Distance measuring apparatus, distance measuring method, reflector and communication system

ABSTRACT

A transmission controller  7 B is configured to transmit an R/W request signal for requesting transmission of a tag response signal to a RFID tag  1  twice. At this time, a frequency controller  7 A controls a PLL section  5 A to transmit the R/W request signal via different carrier frequencies. A phase information acquirer  8 A detects a phase change amount of the tag response signal that is transmitted via different carrier frequencies. A distance calculator  8 B calculates the distance between the reader/writer  2  and the RFID tag  1  on the basis of the phase change amount.

This Application is the National Phase of International Application No.PCT/JP2005/016005 filed Sep. 1, 2005, which designated the U.S. and wasnot published under PCT Article 21(2) in English, and this applicationclaims, via the aforesaid International Application, the foreignpriority benefit of and claims the priority from Japanese ApplicationNo. 2005-066298, filed Mar. 9, 2005, the complete disclosures of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a distance measuring apparatus, adistance measuring method, a reflector, and a communication system,which perform wireless communication with a reflector via radio waves.

BACKGROUND ART

Recently, RFID (Radio Frequency Identification) tags (wireless tags)have been widely used. The RFID tags are used as a substitute forbarcodes. The RFID tags are gathering popularity particularly in thefield of distribution and are expected to spread widely in the nearfuture.

Presently, as a frequency band exclusively for the RFID tags purpose,13.56 MHz band, a so-called UHF band that is somewhere between 800 MHzand 950 MHz, and 2.45 GHz band are used. Among these bands, radio wavesusing the UHF band or the 2.45 GHz band are advantageous over radiowaves using the 13.56 MHz in that it is easy to increase a communicationrange. In addition, the UHF band radio waves are advantageous over the2.4 GHz band radio waves in that it is easy to go behind the object. Forthis reason, RFID tags and reader/writers using the UHF band radio wavesare being developed.

In the case of using the UHF band radio waves, it is possible toincrease the communicable distance between the reader/writer and awireless tag from several ten centimeters to several meters, compared tothe case of using the 13.55 MHz band radio waves which are widely usedat present. Accordingly, by utilizing the UHF band radio waves, itbecomes possible to relatively greatly increase a communication areawhich is a spatial area in which the reader/writer can communicate withthe wireless tags.

Meanwhile, in order to estimate the location of the RFID tags, there hasbeen suggested a technology for measuring the distance between the RFIDtags and a communication station in communication with the RFID tags. Asan example, a technology is known in which a plurality of base stationsreceive signals from an active IC tag as the RFID tag, and the distanceto and location of the active IC tag are estimated on the basis of thereception signals from the active IC tag. In the example, the distancebetween each of the base stations and the active IC tag is estimated onthe basis of the signal intensity of the reception signals from theactive IC tag. That is, the distance estimation is performed byutilizing correlation between the reception signal intensity and thedistance. Also is known a method in which by preparing access points ofwhich the location is known, signals are received simultaneously fromthe active IC tag and the access points, and the delay amount betweenreceiving timings for the respective signals, thereby estimating thedistance to the active IC tag.

As shown in FIG. 26, Japanese Patent Publication No. 2004-507714T(published on Mar. 11, 2004; hereinafter will be referred to as PatentDocument 1) discloses an RF communication system in which aninterrogator 36 as a reader transmits signals 40 and 42 having differentfrequencies to an RF tag 38, and the number of null points in combinedwaves that are obtained by superimposing the two signals onto each otheris counted, thereby estimating the distance between the interrogator 36and the RF tag 38.

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

Although the RFID communication system using the UHF band radio wavesenables a long distance communication, it becomes possible to uselesslycommunicate with an RFID tag that is located far away and does not needto be communicated with. Thus, there are problems in that uselessprocessings are performed to the RFID tag, and the response performancemay be deteriorated by being influenced by multiple paths. In order tosolve such problems, a method can be conceived in which by allowing thereader/writer to calculate the distance to each RFID tag, the RFID tagsthat should be subjected to processes such as analysis or rewriting ofthe data section are chosen. The prior method of calculating thedistance between the reader/writer and the RFID tag has the followingproblems.

First, the estimation of the distance based on the signal intensity ofthe reception signals from the RFID tag or the delay time results in alow measurement precision. The precision obtainable from the abovedistance estimation is actually about in the range of 1 meter to severalmeters. For example, when applied to a distribution management system,such precision is not practical, but a higher distance measurementprecision is required.

In the technology disclosed in Patent Document 1, since the distance isestimated by counting the number of null points in the combined wavesobtained by superimposing the two signal having different frequencies,the distance estimated has very poor precision. More specifically,according to the example described in Paragraph [0025] of PatentDocument 1, when a first signal of 880 MHz and a second signal of 884MHz were used, a first null point was found at a position located about37.5 meters from the interrogator 36 and additional null points werefound every 75 meters from the first null point. For example, if threenull points were found between the interrogator 36 and the RF tag 38,the estimated distance between the interrogator 36 and the RF tag 38 isin the range of 187.5 meter to 262.5 meter. Therefore, 75 meter is anerror range. In the case of occurrence of multiple paths, radio signalsmay propagate to the longer distance, and thus the calculate distancemay become different from the actual distance to the RFID tag.

As shown in FIG. 27, EP1239634A2 (hereinafter will be referred to asPatent Document 2) discloses a communication apparatus 138 thatwirelessly communicates with an RFID tag. Reflection signals from theRFID tag are received through an antenna 140, reception signals inputtedfrom a circulator 92 and carrier signals inputted from a circulator 90and a splitter 98 are multiplied by mixers 100 and 102 to generate an Isignal and an Q signal, and the amplitude and phase of the reflectionsignals from the RFID tag are calculated on the basis of the I and Qsignals. Also is disclosed a method of calculating the distance to theRFID tag by using the phase difference between the carrier signals andthe reflection signals from the RFID tag.

The present invention has been made in view of such problems, and anobject of the invention is to provide a distance measuring apparatus, adistance measuring method, and a communication system capable ofenabling to measure the distance to RFID tags with high precision.Another object of the invention is to provide a distance measuringapparatus, a distance measuring method, a reflector and a communicationsystem capable of enabling to measure the distance to tags without usingspecial RFID tags.

How to Solve the Problems

In order to solve the problems, a distance measuring apparatus accordingto the invention is characterized by comprising:

a transmitter, that transmits a request signal, which is a signalcomposed of one frame, from an antenna to the outside via radio waveshaving a plurality of different carrier frequencies;

a receiver, that receives a reflection signal which is a signal composedof one frame and is obtained when the request signal transmitted fromthe transmitter is reflected by a reflector while being modulated;

a phase information acquirer, that calculates an amount of phase changebetween the reflection signal received by the receiver and the requestsignal for each of the carrier frequencies transmitted from thetransmitter; and

a distance calculator, that calculates a distance between the antennaand the reflector on the basis of the carrier frequencies and the phasechange amount for each of the carrier frequencies acquired by the phaseinformation acquirer.

Here, the reflector is an RFID tag that is equipped with an IC forwireless communication, a storage, and an antenna. Examples of the RFIDtag include a passive RFID tag that does not have a power source such asbattery, operates circuits by drawing energy from radio wavestransmitted from a reader/writer, thus performing wireless communicationwith the reader/writer, and an active RFID tag that has a power sourcesuch as battery. Also, the RFID tag may have its own unique IDinformation.

A distance measuring method according to the invention is characterizedby comprising:

a transmitting step of transmitting a request signal, which is a signalcomposed of one frame, from an antenna to the outside via radio waveshaving a plurality of different carrier frequencies;

a receiving step of receiving a reflection signals which is a signalcomposed of one frame and is obtained when the request signaltransmitted in the transmitting step is reflected from a reflector whilebeing modulated;

a phase information acquisition step of calculating an amount of phasechange between the reflection signal received in the receiving step andthe request signal for each of the carrier frequencies transmitted inthe transmitting step; and

a distance calculating step of calculating a distance between theantenna and the reflector on the basis of the carrier frequencies andthe phase change amount for each of the carrier frequencies acquired inthe phase information acquiring step.

According to the above-mentioned configuration and method, it isconfigured to receive the reflection signal that is transmitted from thereflector via a plurality of different carrier frequencies. Thereflection signal transmitted via the plurality of different carrierfrequencies has different phase states that differ for each of thecarrier frequencies depending on the distance between the reflector thattransmitted the reflection signal and the distance measuring apparatus.

According to the above-mentioned configuration and method, the phasechange amount of the reflection signal is acquired for each of thecarrier frequencies and the distance is calculated on the basis of thephase change amount and the carrier frequencies. In this case, byacquiring the carrier frequencies and the phase change amount of thesignal transmitted via the plurality of carrier frequencies, it becomespossible to calculate the distance with high precision, which will bedescribed in detail later. In other words, according to above-mentionedconfiguration and method, it is possible to provide a distance measuringapparatus capable of calculating the distance with high precision andhigh accuracy.

According to the distance measuring apparatus of the invention, in theabove configuration, the transmitter may set a plurality of dividedperiods in a period for transmitting one request signal, and may controlsuch that the different carrier frequencies are used for the respectivedivided periods.

According to the distance measuring method of the invention, in theabove method, the transmitting step may be configured to set a pluralityof divided periods in a period for transmitting one request signal, andto control such that the different carrier frequencies are used for therespective divided periods.

According to the above-mentioned configuration and method, one requestsignal is transmitted via different carrier frequencies in each of thedivided periods. In this case, the reflection signal is also transmittedvia different carrier frequencies in each of the divided periods.Accordingly, the distance calculator becomes possible to calculate thedistance by analyzing the signal states of one reflection signals foreach of the divided periods. In other words, the distance can becalculated only by transmitting and/or receiving one request signal andone reflection signal. Thus, it becomes possible to reduce the number ofsignal communications required for the distance calculation.Accordingly, it is possible to calculate the distance withoutdeteriorating the communication efficiency.

According to the distance measuring apparatus of the invention, in theabove configuration, the transmitter may transmit the request signal viaone of the carrier frequencies composed of different carrier frequencycomponents.

According to the distance measuring method of the invention, in theabove method, the transmitting step may be configured to transmit therequest signal via one of the carrier frequencies composed of differentcarrier frequency components.

According to the above-mentioned configuration and method, one requestsignal is transmitted via one carrier frequencies composed of differentcarrier frequency components. In this case, the reflection signal isalso transmitted via one carrier frequencies composed of differentcarrier frequency components. Accordingly, the distance calculatorbecomes possible to calculate the distance by analyzing the signalstates of one reflection signal for each of the frequency components. Inother words, the distance can be calculated only by transmitting and/orreceiving one request signal and one reflection signal. Thus, it becomespossible to reduce the number of signal communications required for thedistance calculation. Accordingly, it is possible to calculate thedistance without deteriorating the communication efficiency.

According to the distance measuring apparatus of the invention, in theabove configuration, the reflection signal may be transmitted from thereflector via three or more different carrier frequencies. The phaseinformation acquirer may acquire the phase change amount by selectingsignals of two carrier frequencies having signal states which satisfy aprescribed criterion for calculating the distance, from signals of thecarrier frequencies.

According to the distance measuring method of the invention, in theabove method, the reflection signal may be transmitted from thereflector via three or more different carrier frequencies. The phaseinformation acquirer may acquire the phase change amount by selectingsignals of two carrier frequencies having signal states which satisfy aprescribed criterion for calculating the distance, from signals of thecarrier frequencies.

The case may be considered in which depending on carrier frequencies,the reflection signal may have a signal state that is not suitable forthe distance calculation due to reasons such as occurrence of multiplepaths of the reflection signal. To the contrary, according to theabove-mentioned configuration and method, the signals of two carrierfrequencies of which the signal states satisfy a prescribed criterionfor the distance calculation are selected from signals of three or more,different carrier frequencies, and the phase change amount is detectedon the basis of the signals. Accordingly, it is possible to increase thedetection precision of the phase change amount and thus to increase theprecision of the distance calculation.

In the above configuration, the distance measuring apparatus of theinvention may further comprise a frequency converter, that performs afrequency conversion processing with respect to the reflection signalreceived. The frequency converter may convert the reflection signal toan I signal and a Q signal.

In the above method the distance measuring method of the invention mayfurther comprise a frequency converting step of performing a frequencyconversion processing with respect to the reflection signal received.The frequency converting step converts the reflection signals to an Isignal and a Q signal.

According to the above-mentioned configuration and method, thereflection signal received is subjected to frequency conversion by beingconverted to the I signal and the Q signal. Accordingly, it becomes easyto detect the phase.

According to the distance measuring apparatus of the invention, in theabove configuration, the distance calculator may calculate the distancewith high-resolution spectrum analysis.

According to the distance measuring method of the invention, in theabove method, the distance calculating step may calculate the distancewith high-resolution spectrum analysis.

Conventionally, the high-resolution spectrum analysis has been used forthe purpose of estimating direction of arrival of radio waves byreceiving, as an input, reception signals received through a pluralityof antenna elements. To the contrary, according to the above-mentionedconfiguration and method, the application model of the high-resolutionspectrum analysis is changed by using the reception signal of each ofthe carrier frequencies as a substitute for the reception signalreceived through the antenna elements, used in the conventionalhigh-resolution spectrum analysis for estimation of the direction ofarrival.

In such a high-resolution spectrum analysis, since values for estimationcan be calculated on the basis of the most likely values, even in thecase of occurrence of multiple paths, it is possible to exclude thedistance associated with the multiple paths from consideration. In otherwords, according to the above-mentioned configuration and method, it ispossible to calculate the distance with high accuracy even in theoccurrence of multiple paths.

According to the distance measuring apparatus of the invention, in theabove configuration, the distance calculator may utilize a MUSIC(MUltiple SIgnal Classification) method as the high-resolution spectrumanalysis, in which the reflection signal received via the differentcarrier frequencies is used as an input, an MUSIC evaluation function isobtained using a mode vector as a function of the distance, and thedistance is calculated by obtaining peak values of the MUSIC evaluationfunction.

In the above-mentioned configuration, the distance measuring apparatusof the invention may be configured such that the distance calculatorutilizes an MUSIC (MUltiple SIgnal Classification) method as thehigh-resolution spectrum analysis, in which the reflection signalreceived via the plurality of different carrier frequencies is used asan input of the MUSIC method; and an MUSIC evaluation function isderived using a mode vector as a function of the distance to obtain peakvalues of the MUSIC evaluation function, thereby calculating thedistance.

According to the distance measuring method of the invention, in theabove method, the distance calculator may utilize a MUSIC (MUltipleSignal Classification) method as the high-resolution spectrum analysis,in which the reflection signal received via the different carrierfrequencies is used as an input, an MUSIC evaluation function isobtained using a mode vector as a function of the distance, and thedistance is calculated by obtaining peak values of the MUSIC evaluationfunction.

Although the MUSIC evaluation function generally has only one peakvalue, in the case of occurrence of multiple paths, a plurality of peakvalues may exist. Even in such a case, because the distancecorresponding to the multiple paths is longer than the distance thatshould be calculated, by selecting the shortest of the distancescorresponding to the peak values as the distance to be calculated, itbecomes possible to calculate the distance with high accuracy.

According to the distance measuring apparatus of the invention, in theabove configuration, the distance calculator may calculate the distancealso on the basis of a reception intensity of the reflection signalreceived.

According to the distance measuring method of the invention, in theabove method, the distance calculating step may calculate the distanceon the basis of a reception intensity of the reflection signal received.

According to the above-mentioned configuration and method, the distanceis calculated also on the basis of the reception intensity of thereflection signal. When there are a plurality of distance valuecandidates, for instance, by taking the reception intensity intoconsideration, it becomes possible to select the correct distance andthus to calculate the distance with much higher accuracy.

In the above configuration, the distance measuring apparatus of theinvention may further comprise a reception controller, that acquiresinformation in a data section of the reflection signal, and outputs, tothe outside, the distance information calculated by the distancecalculator which is associated with the information in the data section.

In the above method, the distance measuring method of the invention mayfurther comprise a reception control step of acquiring information in adata section of the reflection signal, and outputting, to the outside,the distance information calculated by the distance calculator which isassociated with the information in the data section.

According to the above-mentioned configuration and method, theinformation in the data section included in the reflection signal givesinformation that identifies the reflector that transmitted thereflection signal, for instance. The reception controller identifies thedistance information measured by the distance calculator and theinformation in the data section which are associated with each other.Accordingly, even when the distance measuring apparatus communicateswith a plurality of reflectors, it becomes possible to distinguish thedistances to the reflectors.

According to the distance measuring apparatus of the invention, in theabove configuration, the distance calculator may measure a directionthat the reflector that has transmitted the reflection signal islocated, on the basis of the reflection signal.

According to the distance measuring method of the invention, in theabove method, the distance calculating step may measure a direction thatthe reflector that has transmitted the reflection signal is located, onthe basis of the reflection signal.

According to the above-mentioned configuration and method, it becomespossible to identify the direction of location where the reflectorexists along with the distance to the reflector. Accordingly, it becomespossible to identify the location where the reflector exists.

According to the distance measuring apparatus of the invention, in theabove configuration, the distance calculator may calculate the distanceby analyzing a signal in a preamble section of the reflection signal.

According to the distance measuring method of the invention, in theabove method, the distance calculating step may calculate the distanceby analyzing a signal in a preamble section of the reflection signal.

According to the above-mentioned configuration and method, the distanceis calculated by analyzing the signal in the preamble section of thereflection signal. Here, the preamble section represents the dataindicating the start of the reflection signal and is composed of aprescribed data component that is common to all reflectors based on thesame standard (EPC, for example). Therefore, the lengths of the preamblesection are all the same for signals that are received from anyreflectors, thus making it possible to perform the signal analysis in anassured manner. Because the signals in the preamble section are all thesame, it becomes possible to calculate the distance even when thesignals had been subjected to PSK modulation.

The respective members in the distance measuring apparatus of theinvention may be realized by a computer. In this case, the invention maybe realized by causing the computer to execute a program that operatesthe computer as the respective members.

A reflector according to the invention is characterized by comprising:

a signal generator, that generates a reflection signal, which is asignal composed of one frame, from a request signal transmitted from thedistance measuring apparatus of the invention; and

a frequency controller, that transmits the reflection signal generatedby the signal generator via a plurality of different carrierfrequencies.

According to the above-mentioned configuration, the reflection signaltransmitted by the reflector is received by the distance measuringapparatus of the invention, thus making the distance measuring apparatuspossible to calculate the distance with high precision and highaccuracy.

According to the reflector of the invention, in the above configuration,the frequency controller may set a plurality of divided periods in aperiod for transmitting one request signal, and controls such that thedifferent carrier frequencies are used for the respective dividedperiods.

According to the above-mentioned configuration, the distance can becalculated only by transmitting one reflection signal. Thus, it becomespossible to reduce the number of signal communications required for thedistance calculation. Accordingly, it is possible to calculate thedistance without deteriorating the communication efficiency.

According to the reflector of the invention, in the above configuration,the frequency controller may transmit the request signal via one of thecarrier frequencies composed of different carrier frequency components.

According to the above-mentioned configuration, the distance can becalculated only by transmitting one reflection signal. Thus, it becomespossible to reduce the number of signal communications required for thedistance calculation. Accordingly, it is possible to calculate thedistance without deteriorating the communication efficiency.

A communication system according to the invention is characterized bycomprising:

the distance measuring apparatus of the invention; and

at least one reflector that performs wireless communication with thedistance measuring apparatus.

According to the above-mentioned configuration, it becomes easy toconstruct a system for managing communication with reflectors andespecially capable of understanding information about distance to thereflectors.

A communication system of the invention is characterized by comprising:

the distance measuring apparatus of the invention; and

a management apparatus, that manages at least one of articles, people,and living things that are associated with the reflectors on the basisof a communication result between the distance measuring apparatus andthe reflectors.

According to the above-mentioned configuration, it becomes easy toconstruct a system that manages articles, people, and living things thatare associated with the reflectors and is capable of identifyinginformation about the location thereof.

ADVANTAGE OF THE INVENTION

As described above, the distance measuring apparatus of the inventionhas a configuration in which the apparatus performs a processing ofreceiving a reflection signal transmitted from a reflector via aplurality of different carrier frequencies, and the apparatus isprovided with a distance calculator that analyzes the reflection signaltransmitted from the reflector to calculate the distance between thereflector and the distance measuring apparatus. With such aconfiguration, it becomes advantageous to calculate the distance betweenthe reflector and the distance measuring apparatus with much higherprecision.

The communication system of the invention has a configuration in whichthe system is provided with the distance measuring apparatus of theinvention and one or more reflectors that perform a wirelesscommunication with the distance measuring apparatus.

With such a configuration, it becomes easy and advantageous to constructa system that manages the communication with the reflectors and is atleast capable of identifying information about the distance to thereflectors.

The communication system of the invention has a configuration in whichthe system is provided with the distance measuring apparatus of theinvention and a management apparatus that manages at least one ofarticles, people, and living things that are associated with thereflectors on the basis of the results of communication between thedistance measuring apparatus and the reflectors.

With such a configuration, it becomes easy and advantageous to constructa system that manages articles, people, and living things that areassociated with the reflectors and is capable of identifying informationabout the location thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a reader/writer of an RFID tagcommunication system according to the invention, showing theconfiguration for measuring the distance between the reader/writer andthe RFID tag.

FIG. 2 is a simplified block diagram showing the configuration of theRFID tag communication system.

FIG. 3A is a diagram showing the state in which an R/W request signaland a tag response signal are communicated between the reader/writer andthe RFID tag.

FIG. 3B is a diagram in which signals transmitted from the reader/writerto the RFID tag and carrier frequencies thereof are illustrated on timeaxis.

FIG. 3C is a diagram in which signals transmitted from the RFID tag tothe reader/writer and carrier frequencies thereof are illustrated ontime axis.

FIG. 4 is a simplified block diagram of the reader/writer, showing thedetailed configuration of a reception processor that enables to performphase detection.

FIG. 5 is a flowchart showing the procedures of a distance measurementprocessing.

FIG. 6 is a flowchart showing the procedures of the distance measurementprocessing based on multiple frequencies.

FIG. 7 is a flowchart showing the procedures of a frequency selectionprocessing and a distance calculation processing.

FIG. 8 is a schematic diagram showing an example of the frequencyselection processing.

FIG. 9A is a diagram showing the state in which multiple paths occur inthe communication between the reader/writer and the RFID tag.

FIG. 9B is a graph showing the change of a MUSIC evaluation functionrelative to distance.

FIG. 9C is a diagram for explaining a processing of estimating thedirection

FIG. 10 is a flowchart showing the first half of the procedures of thedistance measurement processing employing an MUSIC method.

FIG. 11 is a flowchart showing the second half of the procedures of thedistance measurement processing employing the MUSIC method.

FIG. 12A is a diagram showing the case where frequencies are switchedover within one frame, in which signals transmitted from thereader/writer to the RFID tag and carrier frequencies thereof areillustrated on time axis.

FIG. 12B is a diagram showing the case where frequencies are switchedover within one frame, in which signals transmitted from the RFID tag tothe reader/writer and carrier frequencies thereof are illustrated ontime axis.

FIG. 13 is a flowchart showing the procedures of the distancemeasurement processing that involves frequency switching over within oneframe.

FIG. 14 is a simplified block diagram showing the configuration of thereader/writer that enables to identify the RFID tags and the distancesthereof in a linked manner.

FIG. 15 is a flowchart showing the procedures of the distancemeasurement processing based on identification of the RFID tags.

FIG. 16 is a schematic diagram showing the processing of estimating thedirection of location where the RFID tag exists.

FIG. 17 is a simplified block diagram of the reader/writer, showing theconfiguration for calculating the direction.

FIG. 18 is a flowchart showing the first half of the procedures of alocation estimating processing.

FIG. 19 is a flowchart showing the second half of the procedures of thelocation estimating processing.

FIG. 20A is a diagram showing the case where a plurality of frequenciesare simultaneously transmitted within one frame, in which signalstransmitted from the reader/writer to the RFID tag and carrierfrequencies thereof are illustrated on time axis.

FIG. 20B is a diagram showing the case where a plurality of frequenciesare simultaneously transmitted within one frame, in which signalstransmitted from the RFID tag to the reader/writer and carrierfrequencies thereof are illustrated on time axis.

FIG. 21 is a simplified block diagram showing the configuration of thereader/writer when a plurality of frequencies are simultaneouslytransmitted within one frame.

FIG. 22 is a diagram showing an example of a system in which the RFIDtag communication system of the invention is applied to a system forinspecting and/or checking articles in circulation.

FIG. 23A is a diagram showing an example of a system in which the RFIDtag communication system of the invention is applied to a system forsurveillance monitoring of products or stored articles.

FIG. 23B is a diagram showing an example of a system in which the RFIDtag communication system of the invention is applied to an securitysystem in which an RFID tag 1 is attached, for example, to a window or adoor; and the position of the RFID tag 1 is monitored, thereby detectingopening of the window or the door.

FIG. 24 is a diagram showing an example of a system in which the RFIDtag communication system of the invention is applied to the places suchas stations or movie theaters where tickets are examined.

FIG. 25 is a diagram showing an example of a system in which the RFIDtag communication system of the invention is applied to a keyless entrysystem that is suitable for automobiles.

FIG. 26 is a block diagram showing the configuration of an RFcommunication system according to a prior art technology.

FIG. 27 is a block diagram showing the configuration of a communicationapparatus that performs wireless communication with RFID tags inaccordance with a prior art technology

DESCRIPTION OF REFERENCE NUMERALS

-   -   1: RFID TAG    -   2: READER/WRITER    -   3: TRANSMITTER ANTENNA    -   4: RECEIVER ANTENNA    -   4A: FIRST ANTENNA ELEMENT    -   4B: SECOND ANTENNA ELEMENT    -   5: RECEPTION PROCESSOR    -   5A: PLL SECTION    -   5B: MODULATOR    -   5C: POWER AMPLIFIER    -   6: TRANSMISSION PROCESSOR    -   6A, 6A1, 6A2: AMPLIFIER    -   6B: FREQUENCY CONVERTER    -   6B1, 6B2: MIXER    -   6B3: 90-DEGREE PHASE SHIFTER    -   6C: PREAMBLE EXTRACTOR    -   6D: SELECTOR    -   7: COMMUNICATION CONTROLLER    -   7A: FREQUENCY CONTROLLER    -   7B: TRANSMISSION CONTROLLER    -   7C: RECEPTION CONTROLLER    -   8: LOCATION MEASURE    -   8A: PHASE INFORMATION ACQUIRER    -   8B: DISTANCE CALCULATOR    -   8C: DIRECTION CALCULATOR    -   9: EXTERNAL COMMUNICATOR    -   10: AREA DETERMINANT

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, one embodiment of the invention will be described belowwith reference to FIGS. 1 to 25.

Configuration of Reader/Writer

FIG. 2 shows a simplified block diagram of a communication system of theinvention, including RFID tags and a reader/writer. As shown in thedrawing, the communication system is configured to include one or moreRFID tags (reflectors) 1 and a reader/writer (distance measuringapparatus) 2.

The RFID tag 1 is attached to various articles and stores informationabout the article attached or about objects or people associatedtherewith. The RFID tag 1 is configured to include an IC (IntegratedCircuit) for wireless communication, a storage, and an antenna.

In the present embodiment, the RFID tag 1 used is considered as being apassive RFID tag that does not have a power source such as battery,operates circuits by drawing energy from radio waves transmitted fromthe reader/writer 2, thus performing wireless communication with thereader/writer 2. The RFID tag that is usable in the present embodimentis not limited to the passive RFID tag, but an active RFID tag that hasa power source such as battery may be used.

The reader/writer 2 is a device that performs wireless communicationwith each of the RFID tags 1 and reads/writes information from/into theRFID tags 1. In the present embodiment, the reader/writer 2 isconfigured to read/write information from/into the RFID tags 1. However,the invention is not limited to this, and the reader/writer 2 may beconfigured only to read information from the RFID tags 1.

In the present embodiment, the frequency band of the radio wavetransmitted by the reader/writer 2 is in the so-called UHF band that issomewhere between 800 MHz and 960 MHz. By using the radio wave in such afrequency band, the reader/writer 2 becomes able to communication withthe RFID tag 1 that is located in a distance range of from severalmeters to several ten meters. Although the present embodiment isconsidering a UHF band communication, the invention is not limited tothis. As a frequency band that is usable exclusively for RFID tag,frequency bands such as 13.56 MHz band or 2.45 GHz band are available.Additionally, the communication may be made using other frequency bandsas long as wireless communication can be performed in that frequencybands.

The reader/writer 2 is configured to include a transmitter antenna 3, areceiver antenna 4, a transmission processor 5, a reception processor 6,a communication controller 7, a location measure 8, an externalcommunicator 9, an area determinant 10, and an area information storage11.

The transmitter antenna 3 is an antenna that transmits radio waves tothe RFID tags 1, and the receiver antenna 4 is an antenna that receivesradio waves sent back from the RFID tags 1. The transmitter antenna 3and the receiver antenna 4 are configured, for example, as a patchantenna or an array antenna. Although in the present configurationexample, the transmitter antenna 3 and the receiver antenna 4 areseparately provided, a single antenna may be used in which functions ofboth the transmitter antenna 3 and the receiver antenna 4 are integratedinto the antenna.

The transmission processor 5 is a block that performs processes such asmodulation or amplification to the transmission signal to be transmittedfrom the transmitter antenna 3. The reception processor 6 is a blockthat performs processes such as amplification or demodulation to thereception signals received by the receiver antenna 4.

The communication controller 7 is a block that performs a control ofreading and/or writing information from or to the RFID tag 1 forcommunication through the transmitter antenna 3 and/or the receiverantenna 4.

The location measure 8 is a block that measures the location of the RFIDtag 1 on the basis of the reception signals received from the RFID tag1. The measurement of the location of the RFID tag 1 includesmeasurement of the distance between the reader/writer 2 and the RFID tag1; measurement of the direction of the RFID tag 1 as seen from thereader/writer 2; and measurement of the spatial location of the RFID tag1, which will be described in detail later. Strictly speaking, thedistance between the reader/writer 2 and the RFID tag 1 corresponds toan additive average of two distances: the distance between thetransmitter antenna 3 of the reader/writer 2 and the RFID tag 1; and thedistance between the receiver antenna 4 of the reader/writer 2 and theRFID tag 1. Here, if the RFID tag 1 is an active one, the distancecorresponds to the distance between the receiver antenna 4 and the RFIDtag 1.

The area determinant 10 is a block that determines whether the RFID tag1 is located within a prescribed spatial area (communication area) onthe basis of the location measured by the location measure 8. Areainformation about which spatial area is located the communication areais stored in the area information storage 11. The area determinant 10determines whether the RFID tag 1 exists within the communication areaby determining whether the location measured by the location measure 8exists within the communication area as prescribed by the areainformation.

The external communicator 9 is a block that transmits information aboutthe RFID tag 1 read by the reader/writer 2 to an external apparatus orreceives information to be written into the RFID tag 1 from the externalapparatus. The external apparatus and the external communicator 9 arecommunicably connected to each other in a wired or wireless manner. Inthis case, the external apparatus that operates on the basis ofoperations of the reader/writer 2 reading and/or writing informationfrom or into the RFID tag 1 may be configured to include thereader/writer 2.

The area information stored in the area information storage 11 is setdepending on the environment in which the reader/writer 2 is installed.The area information may be set by the external apparatus through theexternal communicator 9, for example. A user interface for inputting thearea information may be provided to the reader/writer 2.

Components of the reader/writer 2, that is, the communication controller7, the location measure 8, the area determinant 10, and the externalcommunicator 9 may be configured as a hardware logic, or may beconfigured to be realized by a processor, such as CPU, executingprograms stored in a storage such as ROM (Read Only Memory) or RAM.

When the above-mentioned configuration are configured by the processorsuch as CPU and the storage, a computer equipped with these means readsout the program from a recording medium and executes the program,whereby various functions and various operations of the communicationcontroller 7, the location measure 8, the area determinant 10, and theexternal communicator 9 are realized. Additionally, by recording theprogram into a removable recording medium, the various functions and thevarious operations can be realized on an arbitrary computer.

The recording medium may be a memory (not shown), for example, a programmedium such as ROM in which a program for execution on a computer isrecorded. Alternatively, the recording medium may be a program mediumthat is readable when being inserted into a program reading device (notshown) that is provided as an external storage device.

In any cases, the program stored is preferably one which can be accessedand executed by a microprocessor. Preferably, the program is read out,and the read-out program is downloaded for execution to a program memoryarea of a microcomputer. In this case, it is assumed that the programfor downloading is prestored in a main apparatus.

If the system is configured to be able to connect to a communicationnetwork including the Internet, the recording medium is preferably onewhich actively downloads a program from the communication network andstores the program therein.

In the case of downloading the program from the communication network,the download program is preferably prestored in the main apparatus, andalternatively the download program may be installed from other recordingmedium.

Configuration Related to Distance Measurement

Next, the configuration of the reader/writer 2, especially for measuringthe distance between the RFID tag 1 and the reader/writer 2 will bedescribed with reference to FIG. 1. As shown in the drawing, thetransmission processor 5 includes a PLL (Phase Locked Loop) section 5Aas a frequency adjuster, a modulator 5B, a power amplifier 5C, and atransmitter 5D. The reception processor 6 includes an amplifier 6A and afrequency converter 6B. The location measure 8 includes a phaseinformation acquirer 8A and a distance calculator 8B. The communicationcontroller 7 includes a frequency controller 7A, a transmissioncontroller 7B, and a reception controller 7C.

In the transmission controller 5, the PLL section 5A is a block thatsets a carrier frequencies of a transmission signal to be transmittedfrom the transmitter antenna 3, and the PLL section 5A is configured bya PLL circuit. The modulator 5B modulates carrier signals generated bythe PLL section 5A and the transmitter 5D to superimpose data to thetransmission signal. In the present embodiment, the modulator 5Bgenerates the transmission signal through ASK (Amplitude Shift Keying)modulation. A modulation method of the transmission signal is notlimited to the ASK modulation, but FSK (Frequency Shift Keying)modulation, PSK (Phase Shift Keying) modulation, or other digitalmodulation methods may be used. The power amplifier 5C is a block thatamplifies the transmission signal.

In the reception processor 6, the amplifier 5A is a block that amplifiesthe reception signals received by the receiver antenna 4. The frequencyconverter 6B is a block that converts the frequency of the receptionsignals amplified by the amplifier 6A into a lower frequency signal.

In the location measure 8, the phase information acquirer 8A is a blockthat detects the amount of phase change in the reception signals that isfrequency-converted by the frequency converter 6B and acquires the phasechange amount as phase information. Here, the phase change amount of thereception signals is the amount of phase change produced when thereception signals propagates a prescribed distance.

More specifically, when the carrier signal outputted from the PLLsection 5A is expressed as sin 2πf₁t, the frequency converter 6Bmultiplies the carrier signal sin 2πf₁t with the reception signals D(t)Asin(2πf₁t+φ) inputted from the amplifier 6A and passes a multiplicationresult value D(t)A cos φ to the phase information acquirer 8A. The phaseinformation acquirer 8A calculates the phase change amount φ on thebasis of the value passed from the frequency converter 6B. In the aboveformula, t is a time; D(t) is a baseband signal when the modulator 5Bperforms ASK modulation; A is an amplitude of the carrier signal itself;φ is the phase change amount when propagating a round-trip distance of2r.

The distance calculator 8B is a block that calculates the distancebetween the RFID tag 1 and the reader/writer 2 on the basis ofinformation about the phase change amount acquired by the phaseinformation acquirer 8A.

In the communication controller, the frequency controller 7A is a blockthat controls the frequency of the transmission signal that is set bythe PLL section 5A. The transmission controller 7B is a block thatinputs data used to modulate the transmission signal to the modulator5B. The reception controller 7C is a block that allows the communicationcontroller 7 b to receive information about the distance calculated bythe distance calculator 8B.

Details of Distance Measurement

Next, details of the distance measurement processing will be described.In the present embodiment, the reader/writer 2 is configured to transmitan R/W request signal (request signal) to the RFID tag 1, and the RFIDtag 1 is configured to reply a response signal (reflection signals) withrespect to the request signal. Such a procedure is illustrated in FIGS.3A to 3C. FIG. 3A is a diagram showing the state in which the R/Wrequest signal and the response signal are communicated between thereader/writer and the RFID tag. FIG. 3B is a diagram in which signalstransmitted from the reader/writer to the RFID tag and carrierfrequencies thereof are illustrated on time axis. FIG. 3C is a diagramin which signals transmitted from the RFID tag to the reader/writer andcarrier frequencies thereof are illustrated on time axis.

The reader/writer 2 is always transmitting a specific signal (signal forpowering the RFID tag 1). When requesting the RFID tag 1 to transmit theresponse signal (hereinafter will be referred to as a tag responsesignal), as shown in FIG. 3B, the reader/writer 2 transmits the R/Wrequest signal that requests a reply of the tag response signal. Thatis, in a normal state, the transmission controller 7B of thereader/writer 2 controls the modulator 5B to transmit data indicative ofthe normal state. When requesting the tag response signal, thereader/writer 2 controls the modulator 5B to transmit data thatconstitutes the R/W request signal. The RFID tag 1 is always monitoringthe signals sent from the reader/writer 2. When detecting reception ofthe R/W request signal, the RFID tag 1 transmits the tag response signalin the form of responding to the R/W request signal.

More specifically, the reader/writer 2 transmits a one-frame signalcomposed of the R/W request signal and a CW (continuous carrier wave).When receiving the R/W request signal and the CW (continuous carrierwave) from the reader/writer 2, the RFID tag 1 transmits the tagresponse signal via a carrier frequencies f₁ corresponding to thefrequency of the CW (continuous carrier wave) to the reader/writer 2. InFIGS. 3B and 3C, the R/W request signal and the CW (continuous carrierwave) are transmitted via the carrier frequencies f₁, and accordingly,the tag response signal is transmitted via the carrier frequencies f₁.

The tag response signal is constituted by a preamble section and a datasection, as illustrated in FIG. 3C. Here, the preamble sectionrepresents the data indicating the start of the reflection signals, andis composed of a prescribed data component that is common to allreflectors based on the same standard (EPC, for example). The datasection is transmitted following the preamble section and represents thedata indicating substantial information that is transmitted from theRFID tag 1. As information included in the data section, ID informationthat is unique to each of the RFID tags 1 can be exemplified. Also,information that should be transmitted from the RFID tag 1, for examplevarious information that is stored in the storage of the RFID tag 1 maybe included in the data section.

The reader/writer 2 transmits the R/W request signal twice and usesdifferent carrier frequencies in each transmission of the R/W requestsignal (more specifically, the CW (continuous carrier wave) followingthe R/W request signal). That is, in the first transmission of the R/Wrequest signal, the frequency controller 7A of the reader/writer 2controls the PLL section 5A to output the carrier signal using a firstfrequency f₁. Meanwhile, in the second transmission of the R/W requestsignal, the frequency controller 7A controls the PLL section 5A tooutput the carrier signal using a second frequency f₂ that is differentfrom the first frequency f₁.

As shown in FIGS. 1, 3B and 3C, when the RFID tag 1 receives the R/Wrequest signal transmitted via the first frequency f₁, the tag responsesignal is replied using the first frequency f₁. In the reader/writer 2,the phase information acquirer 8A analyzes the preamble section of thereceived tag response signal to detect φ₁ that indicates the phasechange amount of the tag response signal. Similarly, when the RFID tag 1receives the R/W request signal transmitted via the first frequency f₂,the tag response signal is replied using the first frequency f₂. In thereader/writer 2, the phase information acquirer 8A analyzes the preamblesection of the received tag response signal to detect φ₂ that indicatesthe phase change amount of the tag response signal.

In the above example, the phase change amount of the tag response signalis detected by analyzing the preamble section. However, the invention isnot limited to this. The phase change amount may be detected whiletaking the data section into consideration. Also, the phase changeamount may be detected for the data section. However, in the case of thePSK modulation, it is difficult to detect the phase change amount thatdepends on distance on the basis of the data section having variablecontents. Thus, it is preferable to detect the phase change amount inthe preamble section having fixed contents.

In this manner, when the phase information acquirer 8A detects the phasechange amounts φ₁ and φ₂, information about the phase change amounts istransferred to the distance calculator 8B. The distance calculator 8Bcalculates the distance between the RFID tag 1 and the reader/writer 2on the basis of φ₁ and φ₂ in the following manner.

First, assuming the distance between the transmitter antenna 3 and theRFID tag 1 is equal to the distance between the receiver antenna 4 andthe RFID tag 1, the distance is defined as r. The phase change amountsφ₁ and φ₂ that are produced when the carrier signals of the firstfrequency f₁ and the second frequency f₂ propagate round-trip distanceof 2r can be expressed by the following formulas.

$\begin{matrix}{{\phi_{1} = {\frac{2\;{\pi f}_{1}}{c}2\; r}}{\phi_{2} = {\frac{2\;\pi\; f_{2}}{c}2\; r}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

In the above formulas, c represents the velocity of light. On the basisof the above two formulas, the distance r can be calculated by thefollowing formula.

$\begin{matrix}{r = \frac{c\;\Delta\;\phi}{4\;\pi{{f_{1} - f_{2}}}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

In this manner, on the basis of the phase change amount φ₁ and φ₂, it ispossible to calculate the distance r between the transmitter antenna 3and the RFID tag 1. Here, it can be expected that a phase shift mayoccur at the RFID tag 1 during the period between reception of the R/Wrequest signal and transmission of the tag response signal. However, thecarrier signals of the first frequency f₁ and the second frequency f₂experience the same amount of the phase shift. Therefore, the phaseshift produced in the RFID tag 1 when communicating signals does notinfluence the distance calculation.

In Formula 2, if φ₂ is equal to or greater than 2π, it is difficult tocalculate the distance r with high accuracy. In other words, a maximummeasurable value r_(max) of the distance r is obtained when Δφ=2π, whichcan be expressed by the following formula.

$\begin{matrix}{r_{\max} = \frac{c}{2{{f_{1} - f_{2}}}}} & \lbrack {{Formula}\mspace{14mu} 3} \rbrack\end{matrix}$

For example, if the difference between the first frequency f₁ and thesecond frequency f₂ is set to 5 MHz, the maximum distance r_(max)becomes 30 meters from Formula 3. Similarly, if the difference betweenthe first frequency f₁ and the second frequency f₂ is set to 2 MHz, themaximum distance r_(max) becomes 75 meters from Formula 3. Since theRFID communication system using UHF band is considering the use of amaximum communicable distance of about 10 meters, such a measurementwill not cause any problems in practical use.

When it is required to measure a distance greater than the maximumdistance r_(max), the measurement of the distance r may be performed inconjunction with the measurement of a reception intensity of thereception signals, for example. Specifically, if there is a possibilitythat Δφ becomes greater than 2π, a candidate r′ of the distance r can beexpressed as r′=r+n·r_(max) (n is an integer of 0 or more). Accordingly,by using the fact that the reception intensity of the reception signalsdecreases as the distance increases, it becomes possible to determinethe value of n.

In the case of using the active RFID tag, the reader/writer 2 does nottransmit the R/W request signal, and thus the distance measurement maybe performed on the basis of the tag response signal that is activelytransmitted from the RFID tag.

Specific Example of Reception Processor

In the distance measurement described above, a processing of detectingthe phase change amount of the reception signals is performed. Detailedconfiguration of the reception processor 6 that enables the detection ofthe phase change amount will be described with reference to FIG. 4. Inthis specific example, the reception processor 6 separates the receptionsignals into an I signal and a Q signal and inputs the I and Q signalsto the location measure 8, thereby enabling the location measure 8 toperform the processing of detecting the phase change amount. As shown inthe drawing, the reception processor 6 includes two amplifiers 6A1 and6A2 as the amplifier 6A, two mixers 6B1 and 6B2 as the frequencyconverter 6B, and a 90-degree phase shifter 6B3.

The reception signals received by the receiver antenna 4 is branchedinto two paths: one is input to the amplifier 6A1; and the other isinput to the amplifier 6A2. The amplifier 6A1 amplifies the inputreception signals and inputs the signal to the mixer 6B1. The amplifier6A2 amplifies the input reception signals and inputs the signal to themixer 6B2.

The mixer 6BG1 multiplies the reception signals inputted from theamplifier 6A1 with the carrier signal outputted from the PLL section 5Ato obtain the I signal and inputs the I signal to the phase informationacquirer 8A. The mixer 6B2 multiplies the reception signals inputtedfrom the amplifier 6A2 with the carrier signal that is outputted fromthe PLL section 5A and 90-degree phase-shifted by the 90-degree phaseshifter 6B3 to obtain the Q signal and inputs the Q signal to the phaseinformation acquirer 8A.

Hereinafter, details of the reception processing and the processing ofcalculating the distance r which are performed by the aboveconfiguration will be described.

Assuming the frequency of the carrier signal is f₁, the signal receivedat the reader/writer 2 after propagating round-trip distance of 2r canbe expressed by the following formula.s ₁(t)=D(t)A sin(2πf ₁ t+φ ₁)  [Formula 4]

In the above formula, t is a time; s₁(t) is a signal state that istransmitted through the carrier signal of a frequency f₁; D(t) is abaseband signal when the modulator 5B performs ASK modulation; A is anamplitude of the carrier signal itself; φ₁ is the phase change amountwhen propagating a round-trip distance of 2r. In this case, I1(t) thatrepresents the signal state of the I signal outputted from the mixer 6B1and Q1(t) that represents the signal state of the Q signal outputtedfrom the mixer 6B2 can be expressed by the flowing formulas.I ₁(t)=D(t)A sin(2πf ₁ t+φ ₁)sin 2πf ₁ t

D(t)A cos φ₁  [Formula 5]Q ₁(t)=D(t)A sin(2πf ₁ t+φ ₁)cos 2πf ₁ t

D(t)A sin φ₁  [Formula 6]From the above formulas, and on the basis of the I signal and the Qsignal, the phase change amount φ₁ attributable to the carrier signal ofthe frequency f₁ can be calculated by the following formula.

$\begin{matrix}{\phi_{1} = {\tan^{- 1}\frac{Q_{1}(t)}{I_{1}(t)}}} & \lbrack {{Formula}\mspace{14mu} 7} \rbrack\end{matrix}$

Similarly, the phase change amount φ₂ attributable to the carrier signalof the frequency f₂ can be calculated by the following formula.

$\begin{matrix}{\phi_{2} = {\tan^{- 1}\frac{Q_{2}(t)}{I_{2}(t)}}} & \lbrack {{Formula}\mspace{14mu} 8} \rbrack\end{matrix}$

In this manner, the phase information acquirer 8A acquires the phasechange amounts φ₁ and φ₂ on the basis of the input I and Q signals.Then, the distance calculator 8B calculates the distance r from thefollowing formula.

$\begin{matrix}{{r = \frac{c\;\Delta\;\phi}{4\;\pi\;{{f_{1} - f_{2}}}}}{{\Theta\;\Delta\;\phi} = {\phi_{1} - \phi_{2}}}} & \lbrack {{Formula}\mspace{14mu} 9} \rbrack\end{matrix}$Procedures in Distance Measurement Processing

Next, the procedure of the distance measurement processing in thereader/writer 2 will be described with reference to the flowchart ofFIG. 5.

First, when the distance measurement processing is started, in Step 1(hereinafter will be referred to as S1), the frequency controller 7Acontrols the PLL section 5A to adjust the frequency of the carriersignal used to transmit the R/W request signal so as to become the firstfrequency f₁.

Next, the transmission controller 7B controls the modulator 5B tosuperimpose the data representing the R/W request signal to the carriersignal. The transmission signal modulated by the modulator 5B isamplified by the power amplifier 5C and is then outputted from thetransmitter antenna 3 (S2). Following the transmission of the R/Wrequest signal, the CW (continuous carrier wave) is transmitted via thefirst frequency f₁ (S3).

When detecting the R/W request signal, the RFID tag 1 transmits, as areply, the tag response signal via a carrier frequencies correspondingto the first frequency f₁ of the CW (continuous carrier wave) that isdetected subsequent to the R/W request signal. When the tag responsesignal is received by the receiver antenna 4, the reception processor 6performs a reception processing to the tag response signal (S4); and thephase information acquirer 8A performs a phase information acquiringprocessing (S5).

That is, in the reception processor 6, on the basis of Formula 4 to 6,the frequency converter 6B calculates the I signal and the Q signal bymultiplying the reception signals inputted from the amplifier 6A and thecarrier signal outputted from the PLL section 5A. When acquiring the Isignal and the Q signal from the frequency converter 6B, on the basis ofFormula 7 and 8, the phase information acquirer 8A calculates the phasechange amounts φ₁ and φ₂ attributable to the first frequency f₁ andstores the amounts and the frequency (first frequency f₁) used as thecarrier signal in a table in a correlated manner.

When the reception processor 6 finishes the reception of the tagresponse signal from the RFID tag 1 (S6), the phase information acquirer8A finishes the phase information acquiring processing (S7). Thereafter,the transmission processor 5 finishes the transmission of the CW(continuous carrier wave), that is, the transmission of the one-framesignal (S8). The reception controller 7C determines whether thereception signals has been received for the entire frequencies. If it isdetermined that the reception signals has not been received for theentire frequencies (NO in S9), the procedure goes back to the processingof S1. In the above example, the first and second frequencies f₁ and f₂are used as a frequency of the reception signals. Therefore, thereception controller 7C determines whether the reception signals hasbeen received for both of the first and the second frequencies f₁ andf₂.

At this moment, since the reception signals is received only for thefirst frequency f₁, the procedure goes back to the processing of S1.When the processing of S1 is performed again, the frequency controller7A controls the PLL section 5A to adjust the frequency of the carriersignal (and the CW (continuous carrier wave)) used to transmit the R/Wrequest signal so as to become the first frequency f₂. Thereafter,processes of S2 to S8 are performed. If it is determined that thereception signals has been received for the entire frequencies (YES inS9), the procedure goes to the processing of S10.

In S10, on the basis of the phase information acquired, the distancecalculator 8B calculates the distance between the RFID tag 1 and thereader/writer 2 in accordance with the above-described methods. Morespecifically, the distance calculator 8B extracts the phase changeamount for each frequency from the table and calculates the distance ron the basis of Formula 9 described above. The distance informationcalculated is transferred to the reception controller 7C. In thismanner, the distance measurement processing is completed.

Distance Measurement with Multiple Frequencies

In the above example, the distance is measured on the basis of thereception signals of two different frequencies. However, as describedlater, the distance may be measured on the basis of the receptionsignals of three or more different frequencies.

When signals are communicated between the reader/writer 2 and the RFIDtag 1, the signal transmitted from the reader/writer 2 directly arrivesat the RFID tag 1 and the signal transmitted from the RFID tag 1directly arrives at the reader/writer 2. However, the case may beconsidered in which the signal does not directly arrive at thereader/writer 2 and the RFID tag 1 but arrives at them after beingreflected from nearby objects (multiple paths). In this case, since thereception signals received at the reader/writer 2 is influenced by themultiple paths, noise components can be mixed into the original phasestate, thus deteriorating the S/N characteristics. That is, in themethod of calculating the distance on the basis of phase information,the precision of the phase information acquired may be degraded, thusdeteriorating the precision of the distance calculated.

In the above example, the method of separating the reception signalsinto the I signal and the Q signal and detecting the phase change amounton the basis of the I and Q signals has been described. However,depending on the phase states, either one of the I signal and the Qsignal may have an extremely smaller signal level than the other one. Inthis case, measurement error attributable to the smaller signal cangreatly influence the calculation of phase. That is, when either one ofthe I signal and the Q signal becomes extremely smaller than the otherone, the phase measurement error increases, thus deteriorating theprecision of the distance calculated.

The above problem can be solved by the following processes. First, thereader/writer 2 transmits the R/W request signal via three or moredifferent frequencies and receives the tag response signal for eachfrequency. Then, among the reception signals, two reception signals ofwhich the S/N ratio and the signal level of the I and Q signals arehigher than those of other reception signals are selected, and thedetection of the phase change amount and the location calculation areperformed on the basis of the two reception signals selected.

Procedures of Distance Measurement Processing with Multiple Frequencies

Next, the procedure of the distance measurement processing with multiplefrequencies in the reader/writer 2 will be described with reference tothe flowchart of FIG. 6.

First, when the distance measurement processing is started, in S11, thefrequency controller 7A controls the PLL section 5A to adjust thefrequency of the carrier signal used to transmit the R/W request signalso as to become the first frequency f₁.

Next, the transmission controller 7B controls the modulator 5B tosuperimpose the data representing the R/W request signal to the carriersignal. The transmission signal modulated by the modulator 5B isamplified by the power amplifier 5C and is then outputted from thetransmitter antenna 3 (S12). Following the transmission of the R/Wrequest signal, the CW (continuous carrier wave) is transmitted via thefirst frequency f₁ (S13).

When detecting the R/W request signal, the RFID tag 1 transmits, as areply, the tag response signal via a carrier frequencies correspondingto the first frequency f₁ of the CW (continuous carrier wave) that isdetected subsequent to the R/W request signal. When the tag responsesignal is received by the receiver antenna 4, the reception processor 6performs a reception processing to the tag response signal (S14); andthe phase information acquirer 8A performs a phase information acquiringprocessing (S15).

That is, in the reception processor 6, on the basis of Formula 4 to 6,the frequency converter 6B calculates the I signal and the Q signal bymultiplying the reception signals inputted from the amplifier 6A and thecarrier signal outputted from the PLL section 5A. When acquiring the Isignal and the Q signal from the frequency converter 6B, on the basis ofFormula 7 and 8, the phase information acquirer 8A calculates the phasechange amounts φ₁ and φ₂ attributable to the first frequency f₁ andderives the signal level s(t) on the basis of Formula 10 describedlater. Then, the phase information acquirer 8A stores the calculatedphase change amounts and the frequency (first frequency f₁) used as thecarrier signal in a table in a correlated manner.s(t)=√{square root over (I(t)² +Q(t)²)}{square root over (I(t)²+Q(t)²)}  [Formula 10]

When the reception processor 6 finishes the reception of the tagresponse signal from the RFID tag 1 (S16), the phase informationacquirer 8A finishes the phase information acquiring processing (S17).Thereafter, the transmission processor 5 finishes the transmission ofthe CW (continuous carrier wave), that is, the transmission of theone-frame signal (S18). The reception controller 7C determines whetherthe reception signals has been received for the entire frequencies(S19). If it is determined that the reception signals has not beenreceived for the entire frequencies, the procedure goes back to theprocessing of S11. In this case, assuming the first to fourthfrequencies are used as a frequency of the reception signals, thereception controller 7C determines whether the reception signals hasbeen received for the entire, first to fourth frequencies.

At this moment, since the reception signals is received only for thefirst frequency f₁, the procedure goes back to the processing of S11.When the processing of S11 is performed again, the frequency controller7A controls the PLL section 5A to adjust the frequency of the carriersignal (and the CW (continuous carrier wave)) used to transmit the R/Wrequest signal so as to become the first frequency f₂. Thereafter,processes of S12 to S18 are repeated until the reception of thereception signals is confirmed for the fourth frequency f₄. If it isdetermined in S19 that the reception signals has been received for theentire frequencies, the procedure goes to the processing of S20.

In S20, the frequency selection processing is performed by the phaseinformation acquirer 8A; and the distance calculation processing isperformed by the distance calculator 8B, thereby finishing the distancemeasurement processing.

Next, the procedure of the frequency selection processing in S20 in thephase information acquirer 8A and the procedure of the distancecalculation processing in S20 in the distance calculator 8B will bedescribed with reference to the flowchart of FIG. 7.

First, in S21, the phase information acquirer 8A acquires the signallevel of the reception signals for the entire frequencies received fromthe table.

Next, in S22, the phase information acquirer 8A determines whether thesignal level of the reception signals for each frequency is greater thana prescribed threshold. The prescribed threshold is preset as small aspossible so as to provide sufficient precision to the distancecalculation. The phase information acquirer 8A determines whether thenumber of frequencies of the reception signals that are greater than theprescribed threshold is less than 2; more than 2; or 2.

If the number of frequencies of the reception signals that are greaterthan the prescribed threshold is less than 2, in S23, the phaseinformation acquirer 8A instructs the communication controller 7 totransmit the R/W request signal via other frequencies in order toreacquire the reception signals at other frequencies.

If the number of frequencies of the reception signals that are greaterthan the prescribed threshold is more than 2, first, in S24, the phaseinformation acquirer 8A removes the frequencies of the reception signalsthat are not greater than the prescribed threshold from selectioncandidates. Next, in S25, the phase information acquirer 8A extracts,for each of the frequencies of the reception signals that are left asthe selection candidate, one of the I signal and the Q signal that hasthe smaller signal level, and uses the smaller one as a minimalcomponent. Thereafter, in S25, the phase information acquirer 8A selectstwo minimal components of which the signal level is greater than that ofother minimal components of other frequencies. For the reception signalsof the frequency corresponding to the two selected minimal components,the phase information acquirer 8A acquires the phase information fromthe table and transfers the information to the distance calculator 8B.In S27, the distance calculator 8B performs the distance calculationprocessing on the basis of the phase information received.

Meanwhile, if the number of frequencies of the reception signals thatare greater than the prescribed threshold is 2, the phase informationacquirer 8A acquires the phase information of the two reception signalsand transfers the information to the distance calculator 8B. In S27, thedistance calculator 8B performs the distance calculation processing onthe basis of the phase information received.

FIG. 8 schematically shows an example of the frequency selectionprocessing. In this example, the case is considered in which thereception signals is received via frequencies f₁ to f₄. In the case ofthe reception signals of frequency f₁, the I signal component isrepresented by I₁; the Q signal component is represented by Q₁; and thereception signals level is represented by S₁. In the case of thereception signals of frequency f₂, the I signal component is representedby I₂; the Q signal component is represented by Q₂; and the receptionsignals level is represented by S₂. In the case of the reception signalsof frequency f₃, the I signal component is represented by I₃; the Qsignal component is represented by Q₃; and the reception signals levelis represented by S₃. In the case of the reception signals of frequencyf₄, the I signal component is represented by I₄; the Q signal componentis represented by Q₄; and the reception signals level is represented byS₄.

First, in S22, among the signal levels S₁ to S₄, it is determined thatS₂ is smaller than the prescribed threshold. Thus, in S24, the receptionsignals of frequency f₂ is removed from the selection candidates. Next,in S25, Q₁, I₃, and Q₄ are selected as the minimal component forfrequencies f₁, f₃, and f₄, respectively. In S25, among Q₁, I₃, and Q₄,Q₁ and I₃ that have greater signal level are selected, and thus thereception signals of frequencies f₁ and f₃ are selected.

Through the above processes, it becomes possible to select the receptionsignals of two frequencies for use in the distance calculation whileexcluding, from consideration, the reception signals that hasdeteriorated S/N characteristics by being influenced by the multiplepaths and the reception signals in which either one of the I signal andthe Q signal has an extremely smaller signal level that the other one.Accordingly, it becomes possible to maintain the precision of thedistance calculation at a high level under any circumstance.

Distance Calculation Method Employing MUSIC Method

Next, another example of the distance calculating method will bedescribed. In the above example, the phase change amounts of thereception signals are detected for two frequencies, and the distance ris calculated by Formula 9 on the basis of the phase change amounts. Tothe contrary, as described later, the distance r may be calculated byemploying the thinking method of an MUSIC (MUltiple SignalClassification) method, which is one of the high-resolution spectrumanalysis methods.

The MUSIC method has been widely used as a method of estimating thedirection of arrival of radio waves. In the MUSIC method, the directionof arrival of radio waves is estimated by analyzing the receptionsignals received through a plurality of antenna elements. The distance rcan be estimated using the MUSIC method by replacing the receptionsignals received through each of the antenna elements for estimation ofthe direction of arrival with the reception signals of each frequency soas to transform the application model (a mode vector a(θ_(i)) used inthe estimation of the direction of arrival) (see Formula 11) in theMUSIC method into a mode vector a(r_(i)) used in the estimation of thedistance (see Formula 12). By employing the MUSIC method in theestimation of the distance r, as described later, it becomes possible tofurther reduce the effect of multiple paths under real environmentinvolving the occurrence of multiple paths and thus to further increasethe precision.

That is, assuming the number of array antenna elements is K; thewavelength of incoming waves is λ; the number of incoming waves is L;and the arrival angle of the i-th incoming waves is θ_(i) (I=1 to L), anarray response vector a(θ_(i)) for the i-th incoming waves can beexpressed as:a(θ_(i))=[exp{jΦ ₁(θ_(i))}, . . . , exp{jΦ _(K)(θ_(i))}]^(T);Φ_(N)(θ_(i))=−(2π/λ)d _(N) sin(θ_(i))  (see Formula 11).In the above formula, Φ_(N)(θ_(i)) is a reception phase of the i-theincoming waves in the N-th antenna element; T is a transposition; andd_(N) is the location of the N-th antenna element.

In Formula 11, the mode vector a(r_(i)) (see Formula 12) can be derivedby replacing the number K of array antenna elements with the number ofused frequencies (f₁, f₂, f₃, . . . , and f_(K)); the arrival angleθ_(i) of the i-th incoming waves with the distance r_(i)(r₁ to r_(L)) tothe i-th tag; the array response vector a(θ_(i)) for the i-th incomingwaves with the array response vector a(r_(i)) for the i-th tag; and thereception phase Φ_(N)(θ_(i)) of the i-th incoming waves in the N-thantenna element with the reception phaseΦ_(N)(r_(i))(Φ_(N)(r_(i))=−2πf_(N)·2r_(i)/c; c is the speed of light(3*10⁸) of the signal received from the i-th tag via the N-th frequency(see FIG. 9C).

$\begin{matrix}{{a( \theta_{i} )} = \lbrack {{\exp( {{- j}\frac{2\;\pi}{\lambda}{\mathbb{d}_{1}\sin}\;\theta_{i}} )},\Lambda,{\exp( {{- j}\frac{2\;\pi}{\lambda}{\mathbb{d}_{N}\sin}\;\theta_{i}} )}} \rbrack^{T}} & \lbrack {{Formula}\mspace{14mu} 11} \rbrack \\{{a( r_{i} )} = \lbrack {{\exp( {{- {j2}}\;\pi\; f_{1}\frac{2\; r_{i}}{c}} )},\Lambda,{\exp( {{- {j2}}\;\pi\; f_{N}\frac{2\; r_{i}}{c}} )}} \rbrack^{T}} & \lbrack {{Formula}\mspace{14mu} 12} \rbrack\end{matrix}$

Hereinafter, details of the distance measurement processing employingthe MUSIC method (hereinafter will be referred to as a distanceestimation MUSIC method) will be described. The distance measurementprocessing described later is performed by the location measure 8.

In the case of the reception signals of frequency f₁, I₁(t) thatrepresents the state of the I signal and Q₁(t) that represents the stateof the Q signal are expressed by Formula 5 and Formula 6, respectively.Here, x₁(t) that represents the reception signals of frequency f₁ as acomplex representation can be expressed by the following formula.x ₁(t)=D(t)A[I ₁(t)+jQ ₁(t)]  [Formula 13]

Similarly, x_(N)(t) that represents the reception signals of frequencyf_(N) as a complex representation can be expressed by the followingformula.x _(N)(t)=D(t)A[I _(N)(t)+jQ _(N)(t)]  [Formula 14]

Considering the case in which the reception signals are received via Kfrequencies, a correlation matrix R_(XX) can be produced on the basis ofthe reception signals of frequencies f₁ to f_(K) as follows.R _(xx) =E[X(t)X(t)]^(H)ΘX(t)=[x ₁(t),x ₂(t),Λ,x _(K)(t)]  [Formula 15]

In the above formula, H is a complex conjugate transposition; and E[ ]is a temporal average. Next, eigen value decomposition is applied to thecorrelation matrix R_(xx) obtained above by the following formula.R _(xx) e _(i)=(μ_(i)+σ²)e _(i)  [Formula 16]

In the above formula, e_(i) is an eigenvector of R_(xx); μ_(i) is aneigen value; and σ² is a noise power. From the above facts, thefollowing relation is satisfied.e _(i) ^(H) a(r)=0  [Formula 17]here, 1≦i≦L

$\begin{matrix}{{a( r_{i} )} = \lbrack {{\exp( {{- {j2}}\;\pi\; f_{1}\frac{2\; r_{i}}{c}} )},\Lambda,{\exp( {{- {j2}}\;\pi\; f_{K}\frac{2\; r_{i}}{c}} )}} \rbrack^{T}} & \lbrack {{Formula}\mspace{14mu} 18} \rbrack\end{matrix}$

From the above facts, the mode vector and MUSIC evaluation functionP_(MUSIC) in the distance estimation MUSIC method is given as follows.

$\begin{matrix}{{P_{MUSIC}(r)} = \frac{{a^{H}(r)}{a(r)}}{\sum\limits_{i = {L + 1}}^{K}\;{{e_{i}^{H}{a(\theta)}}}^{2}}} & \lbrack {{Formula}\mspace{14mu} 19} \rbrack\end{matrix}$

In the above formula, by varying the distance r, the graph as shown inFIG. 9B is obtained. In the graph, the horizontal axis represents thedistance r, and the vertical axis represents the evaluation functionP_(MUSIC). As shown in the graph, the evaluation function P_(MUSIC)includes a peak value and the value of r corresponding to the peak valuecorresponds to the distance r to be calculated.

In the graph shown in FIG. 9B, although the evaluation functionP_(MUSIC) includes only one peak value, other peak values may exist.This is because with the influence of multiple paths, the peak valuesare produced at distances corresponding to the multiple paths. Even insuch a case, because the distances corresponding to the multiple pathsare longer than the distance r that should be calculated, by selectingthe shortest of the distances corresponding to the peak values as thedistance to be calculated, it becomes possible to calculate the distancer with high accuracy.

In the above example, although the MUSIC method as the high-resolutionspectrum analysis method is employed in the distance measurement, otherhigh-resolution spectrum analysis methods such as Beam former method,Capon method, LP (Linear Prediction) method, Min-Norm method, or ESPRITmethod may be employed in the distance measurement.

Procedures of Distance Measurement Processing Employing MUSIC Method

Next, the procedures of the distance measurement processing employingthe MUSIC method in the reader/writer 2 will be described with referenceto the flowcharts of FIGS. 10 and 11.

First, when the distance measurement processing is started, in S31, thefrequency controller 7A controls the PLL section 5A to adjust thefrequency of the carrier signal used to transmit the R/W request signalso as to become the first frequency f₁.

Next, the transmission controller 7B controls the modulator 5B tosuperimpose the data representing the R/W request signal to the carriersignal. The transmission signal modulated by the modulator 5B isamplified by the power amplifier 5C and is then outputted from thetransmitter antenna 3 (S32). Following the transmission of the R/Wrequest signal, the CW (continuous carrier wave) is transmitted via thefirst frequency f₁ (S33).

When detecting the R/W request signal, the RFID tag 1 transmits, as areply, the tag response signal via a carrier frequencies correspondingto the first frequency f₁ of the CW (continuous carrier wave) that isdetected subsequent to the R/W request signal. When the tag responsesignal is received by the receiver antenna 4, the reception processor 6performs a reception processing to the tag response signal (S34); andthe phase information acquirer 8A performs a phase information acquiringprocessing (S35).

That is, in the reception processor 6, on the basis of Formulas 4 to 6,the frequency converter 6B calculates the I signal and the Q signal bymultiplying the reception signals inputted from the amplifier 6A and thecarrier signal outputted from the PLL section 5A. When acquiring the Isignal and the Q signal from the frequency converter 6B, on the basis ofFormulas 13 and 14, the phase information acquirer 8A calculates x₁(t)that represents the reception signals of the first frequency f₁ as acomplex representation and stores the complex representation x₁(t) andthe frequency (first frequency f₁) used as the carrier signal in a tablein a correlated manner.

When the reception processor 6 finishes the reception of the tagresponse signal from the RFID tag 1 (S36), the phase informationacquirer 8A finishes the phase information acquiring processing (S37).Thereafter, the transmission processor 5 finishes the transmission ofthe CW (continuous carrier wave), that is, the transmission of theone-frame signal (S38). The reception controller 7C determines whetherthe reception signals has been received for the entire frequencies. Ifit is determined that the reception signals has not been received forthe entire frequencies, the procedure goes back to the processing ofS31. In this case, assuming the first to N-th frequencies are used as afrequency of the reception signals, the reception controller 7Cdetermines whether the reception signals has been received for theentire, first to N-th frequencies.

At this moment, since the reception signals is received only for thefirst frequency f₁, the procedure goes back to the processing of S31.When the processing of S31 is performed again, the frequency controller7A controls the PLL section 5A to adjust the frequency of the carriersignal (and the CW (continuous carrier wave)) used to transmit the R/Wrequest signal so as to become the first frequency f₂. Thereafter,processes of S32 to S38 are repeated until the reception of thereception signals is confirmed for the N-th frequency. If it isdetermined in S39 that the reception signals has been received for theentire frequencies, the procedure goes to the processing of S40.

In S40, the location measure 8 reads the reception signals x_(n)(t) ofeach frequency from the table and creates the correlation matrix R^(xx)on the basis of the reception signals of each frequency. Next, thelocation measure 8 applies the eigen value decomposition to thecorrelation matrix R_(xx) (S40) and obtains spectra of the MUSICevaluation function P_(MUSIC) so as to search peak values (S42). In thismanner, the distance r is calculated in S43.

Frequency Switching within One Frame

In the above example, the reader/writer 2 is configured to transmit theR/W request signal a plurality of times while changing the carrierfrequencies in each transmission and to receive the tag response signalof which the frequencies of the carrier signal are different from eachother. To the contrary, the reader/writer 2 may be configured tocalculate the distance such that the carrier frequencies is changed morethan one times in the course of transmitting the R/W request signal andCW (continuous carrier wave) that is composed of one frame, and the tagresponse signal that is composed of one frame is received with thecarrier frequencies changed more than one times in the midway, therebycalculating the distance on the basis of the tag response signal.

The reader/writer 2 is always transmitting a specific signal. Whenrequesting the RFID tag 1 to transmit the tag response signal, as shownin FIG. 12A, the reader/writer 2 transmits the R/W request signal thatrequests a reply of the tag response signal. The frequency controller 7Asets a plurality of divided periods in the transmission period of theR/W request signal (more specifically, in the transmission period of theCW (continuous carrier wave) following the R/W request signal) andcontrols the PLL section 5A such that different carrier frequencies areused for each of the divided periods. In the example shown in FIG. 12A,three divided periods are set such that frequency f₁ is used in thefirst divided period, frequency f₂ is used in the second divided period,and frequency f₃ is used in the third divided period.

The RFID tag 1 is always monitoring the signals sent from thereader/writer 2. When detecting reception of the R/W request signal, theRFID tag 1 transmits the tag response signal in the form of respondingto the R/W request signal. In this case, the carrier frequencies of thetag response signal are changed with the temporal change of the carrierfrequencies of the R/W request signal (more specifically, the CW(continuous carrier wave) following the R/W request signal). In theexample shown in FIG. 12B, the carrier frequencies of the tag responsesignal are configured such that frequency f₁ is used in the firstperiod, frequency f₂ is used in the second period, and frequency f₃ isused in the first period.

By receiving such tag response signal, the location measure 8 becomespossible to detect the states of the reception signals transmitted via aplurality of different frequencies. In the reception processor 6, thetimings for changing the frequencies of the tag response signal are seton the basis of the frequency switching over timings at the time oftransmitting the R/W request signal (more specifically, the CW(continuous carrier wave) following the R/W request signal).

Here, the period in which the carrier frequencies of the tag responsesignal are switched is preferably set within the period of the preamblesection. This is because the length of the data section varies in thetag response signal, but the length of the preamble section is fixed.Thus, it becomes possible to secure the period in which the carrierfrequencies are switched.

Procedures of Distance Measurement Processing Involving FrequencySwitching within One Frame

Next, the procedures of the distance measurement processing thatinvolves frequency switching over within one frame in the reader/writer2 will be described with reference to the flowchart of FIG. 13.

First, when the distance measurement processing is started, thefrequency controller 7A controls the PLL section 5A such that thefrequencies of the carrier signal (more specifically, the CW (continuouscarrier wave) following the R/W request signal) to be used at the timeof transmitting the R/W request signal are switched over for each of theplurality of divided periods. Next, the transmission controller 7Bcontrols the modulator 5B to superimpose the data representing the R/Wrequest signal to the carrier signal. The transmission signal modulatedby the modulator 5B is amplified by the power amplifier 5C and is thenoutputted from the transmitter antenna 3 (S51). Following thetransmission of the R/W request signal, the CW (continuous carrier wave)is transmitted via different frequencies for each of the plurality ofdivided periods (S53, S55, S58). Accordingly, the R/W request signal(more specifically, the CW (continuous carrier wave) following the R/Wrequest signal) is transmitted in which frequencies are switched over ineach of the divided periods.

When detecting the R/W request signal, the RFID tag 1 transmits, as areply, the tag response signal via a carrier frequencies (frequenciesf₁, f₂, and f₃) corresponding to the time-varying carrier frequencies(frequencies f₁, f₂, and f₃) of the CW (continuous carrier wave) that isdetected subsequent to the R/W request signal. When the tag responsesignal is received by the receiver antenna 4, the reception processor 6performs a reception processing to the tag response signal (S53, S56,S59); and the phase information acquirer 8A performs a phase informationacquiring processing (S54, S57, S60). In this case, the carrierfrequencies of the tag response signal received are changed with thetemporal change of the carrier frequencies of the R/W request signal(more specifically, the CW (continuous carrier wave) following the R/Wrequest signal).

That is, in the reception processor 6, the frequency converter 6Bidentifies the preamble section from the tag response signal, andcalculates the I signal and the Q signal for each frequency of thedivided periods in the preamble section on the basis of Formulas 4 to 6.When acquiring the I signal and the Q signal for each frequency from thefrequency converter 6B, on the basis of Formula 7 and 8, the phaseinformation acquirer 8A calculates the phase change amounts φ₁ and φ₂for each frequency and stores the amounts and the frequency used as thecarrier signal in a table in a correlated manner. In this case, theswitching over timings of the divided periods are set on the basis ofthe frequency switching over timings at the time of transmitting the R/Wrequest signal (more specifically, the CW (continuous carrier wave)following the R/W request signal).

In the reception processor 6 and the phase information acquirer 8A, thefrequency switching processing and the phase information acquiringprocessing for each frequency (frequencies f₁, f₂, f₃) of the dividedperiods in the preamble section of the tag response signal are performedwhen the transmission processor 5 is transmitting the CW (continuouscarrier wave) of a corresponding frequency. For example, when thetransmission processor 5 starts transmission of the CW (continuouscarrier wave) at the first frequency f₁ (S52), the RFID tag 1 transmits,as a reply, the signal corresponding to the first divided period in thepreamble section of the tag response signal via the first frequency f₁.The reception processor 6 receives the tag response signal of thefrequency f₁ (S53), and the phase information acquirer 8A acquires thephase information of the frequency f₁ (S54). After a prescribed period,the transmission processor 5 starts transmission of the CW (continuouscarrier wave) at the second frequency f₂ (S55). When receiving the CW(continuous carrier wave) of the second frequency f₂, the RFID tag 1transmits, as a reply, the signal corresponding to the subsequentdivided period in the preamble section of the tag response signal viathe second frequency f₂. The reception processor 6 receives the tagresponse signal of the frequency f₂ (S56), and the phase informationacquirer 8A acquires the phase information of the frequency f₂ (S57).After a prescribed period, the transmission processor 5, the receptionprocessor 6, and the phase information acquirer 8A performs the sameoperations with respect to the third frequency f₃ (S58 to S60).

When the reception processor 6 finishes the reception of the tagresponse signal from the RFID tag 1 (S61), the transmission processor 5finishes the transmission of the CW (continuous carrier wave), that is,the transmission of the one-frame signal (S62). Thereafter, the distancecalculator 8B calculates the distance r on the basis of the phaseinformation of the plurality of different frequencies stored in thetable, thereby completing the processing (S63).

In the above example, the distance r is calculated on the basis ofphase. However, the same can be applied to the distance measurementprocessing employing the MUSIC method. In addition, there may beconfigured such that two reception signals of which the S/N ratio andthe signal level of the I and Q signals are higher than those of otherreception signals are selected, and the detection of the phase changeamount and the location calculation are performed on the basis of thetwo reception signals selected.

Identification of Distance to RFID Tags

As described above, the data section is included in the tag responsesignal. When the data section contains ID information that is unique toeach RFID tag 1, it becomes possible to identify the distance measuredusing the tag response signal in the above-described manner and the RFIDtag 1 that transmitted the tag response signal in a linked manner.Hereinafter, the configuration for enabling such identification will bedescribed with reference to FIG. 14.

The configuration shown in FIG. 14 is different from that of FIG. 4 inthat the reception processor 6 of FIG. 14 is provided with a preambleextractor 6C. Other configurations are the same as those of FIG. 4, andthus descriptions thereof will be omitted.

The preamble extractor 6C receives the I signal and the Q signal thatare outputted from the mixer 6B1 and the mixer 6B2, respectively toextract the preamble section of the tag response signal, and transfersthe preamble section to the location measure 8. At this time, the datasection of the tag response signal is transferred as a reception frameto the reception controller 7C of the communication controller 7. Thelocation measure 8 analyzes the preamble section to measure the distancein the above-described manner and transfers the measurement informationto the reception controller 7C.

The reception controller 7C identifies the ID information of the RFIDtag 1 that transmitted the tag response signal by analyzing the datasection received from the preamble extractor 6C. The receptioncontroller 7C identifies the measurement result of the distance to theRFID tag 1 measured by the location measure 8 and the ID information ina linked manner. Accordingly, even when the RFID tag communicationsystem communicates with a plurality of RFID tags 1, it becomes possibleto distinguish the distance to each of the RFID tags 1.

The information that is identified along with the distance informationin the linked manner is not limited to the ID information that is uniqueto each of the RFID tags 1. Other information may be linked to thedistance information as long as the information is described in the datasection of the tag response signal.

Procedures of Distance Measurement Processing with Identification ofRFID Tags

Next, the procedures of the distance measurement processing withidentification of the RFID tags in the reader/writer 2 will be describedwith reference to the flowchart of FIG. 15.

First, when the distance measurement processing is started, in S71, thefrequency controller 7A controls the PLL section 5A to adjust thefrequency of the carrier signal used to transmit the R/W request signalso as to become the first frequency f₁.

Next, the transmission controller 7B controls the modulator 5B tosuperimpose the data representing the R/W request signal to the carriersignal. The transmission signal modulated by the modulator 5B isamplified by the power amplifier 5C and is then outputted from thetransmitter antenna 3 (S72). Following the transmission of the R/Wrequest signal, the CW (continuous carrier wave) is transmitted via thefirst frequency f₁ (S73).

When detecting the R/W request signal, the RFID tag 1 transmits, as areply, the tag response signal via a carrier frequencies correspondingto the first frequency f₁ of the CW (continuous carrier wave) that isdetected subsequent to the R/W request signal. When the tag responsesignal is received by the receiver antenna 4, the reception processor 6performs a reception processing to the tag response signal (S74); andthe phase information acquirer 8A performs a phase information acquiringprocessing (S75).

That is, in the reception processor 6, on the basis of Formulas 4 to 6,the frequency converter 6B calculates the I signal and the Q signal bymultiplying the reception signals inputted from the amplifier 6A and thecarrier signal outputted from the PLL section 5A. The preamble extractor6C extracts the preamble section of the received tag response signal (Isignal and Q signal) and transfers the preamble section to the locationmeasure 8. At this time, the data section of the tag response signal istransferred to the reception controller 7C. When receiving the preamblesection from the preamble extractor 6C, on the basis of Formula 7 and 8,the location measure 8 calculates the phase change amounts φ₁ and φ₂attributable to the first frequency f₁ and stores the amounts and thefrequency (first frequency f₁) used as the carrier signal in a table ina correlated manner (S75).

When the reception processor 6 finishes the reception of the tagresponse signal from the RFID tag 1 (S76), the location measure 8finishes the phase information acquiring processing (S77). Thereafter,the transmission processor 5 finishes the transmission of the CW(continuous carrier wave) (S78). The reception controller 7C determineswhether the reception signals has been received for the entirefrequencies. If it is determined that the reception signals has not beenreceived for the entire frequencies (NO in S79), the procedure goes backto the processing of S71. If it is determined that the reception signalshas been received for the entire frequencies (YES in S79), the proceduregoes to the processing of S80.

In S80, the location measure 8 extracts the phase information of eachfrequency from the table to calculate the distance (S80). The locationmeasure 8 may calculate the distance by using the above-described MUSICmethod as well as on the basis of the phase information.

Meanwhile, when receiving the data section from the preamble extractor6C, the reception controller 7C verifies the ID information of the RFIDtag 1 that transmitted the tag response signal on the basis of the datasection (S81). Then, the reception controller 7C registers the distanceinformation received from the location measure 8 and the ID informationof the RFID tag 1 in a linked manner (S82). The combined information ofthe distance and the ID information of the RFID tag 1 is registered in arecording section (not shown) provide in the communication controller 7and is then transmitted to an external apparatus through the externalcommunicator 9 shown in FIG. 2. In this manner, the distance measurementprocessing is completed.

Location Estimating Processing

In the above example, measurement of the distance to each of the RFIDtags 1 has been described. The direction of location where each RFID tag1 exists as seen from the reader/writer 2 may be measured. With suchmeasurement, it becomes possible to specify the distance to and thedirection of each RFID tag 1 and thus to specify the location where eachRFID tag 1 exist. As a method of estimating the direction of locationwhere the RFID tag 1 exists, a method can be used in which the pluralityof antenna elements of the receiver antenna 4 are arranged in an arrayconfiguration, and the difference between phases of the signals receivedat each of the antenna elements is detected. Hereinafter, a processingof estimating the direction of location where the RFID tag 1 exists willbe described.

FIG. 16 is a schematic diagram showing the processing of estimating thedirection of location where the RFID tag exists. In the drawing, thereceiver antenna 4 is constituted by two antenna elements: a firstantenna element 4A; and a second antenna element 4B. In addition, θdesignates an angle indicating the direction of location where the RFIDtag 1 exists. This angle θ is measured in a state in which a normal linedirection of a plane including both of radio wave reception points atthe first antenna element 4A and the second antenna element 4B is set to0 degree.

Assuming the distance between the radio wave reception points at thefirst antenna element 4A and the second antenna element 4B is d, a phasedifference Δφ of signals received at the first antenna element 4A andthe second antenna element 4B is represented by the following formula.

$\begin{matrix}{{{\Delta\;\phi} = {{k \cdot d}\;\sin\;\theta}}{{\Theta\; k} = \frac{2\;\pi}{\lambda}}} & \lbrack {{Formula}\mspace{14mu} 20} \rbrack\end{matrix}$

Here, assuming d=λ/2, the phase difference Δφ is represented by thefollowing formula.Δφ=π sin θ  [Formula 21]

Thus, on the basis of a phase difference Δφ, the direction of existentlocation θ is represented by the following formula.

$\begin{matrix}{\theta = {\sin^{- 1}\frac{\Delta\;\phi}{\pi}}} & \lbrack {{Formula}\mspace{14mu} 22} \rbrack\end{matrix}$

That is, it is possible to obtain the direction of existent location θby obtaining the phase difference Δφ.

FIG. 17 shows the configuration of the reader/writer 2 related to thecalculation of direction. The configuration shown in the drawing isdifferent from that shown in FIG. 4 in that in FIG. 17, a directioncalculator 8C is provided to the location measure 8, and a selector 6Dis provided to the reception processor 5. Other configurations are thesame as the configurations shown in FIG. 4, and thus descriptionsthereof will be omitted.

The selector 6D selectively switches over the signals received by thefirst antenna element 4A and the second antenna element 4B in thereceiver antenna 4, and transfers the signals to the amplifier 6A1 andthe amplifier 6A2. The selecting operation of the selector 6D iscontrolled by the reception controller 7C.

The direction calculator 8C acquires information about the phasedifference between the signal received by the first antenna element 4Aand the signal received by the second antenna element 4B from the phaseinformation acquirer 8A, and calculates the direction of existentlocation θ of the RFID tag 1 on the basis of the phase differenceinformation by the above-mentioned processing. Then, the receptioncontroller 7C acquires the distance information calculated by thedistance calculator 8B and information about the direction of existentlocation calculated by the direction calculator 8C, and transfers thisinformation to the area determinant 10.

The area determinant 10 determines whether the RFID tag 1 is locatedwithin a prescribed spatial area (communication area) on the basis ofthe distance information and the information about the direction ofexistent location as the location information. At this time, the areadeterminant 10 determines whether the RFID tag 1 exists within thecommunication area on the basis of the area information that is storedin the area information storage 11.

As a method of obtaining the direction of existent location θ of theRFID tag 1, it is not limited to the above-mentioned method, and it ispossible to use various known methods. For example, as technology ofestimating the direction of arrival (DOA: Direction of Arrival) of radiowaves, Beam former method, Capon method, LP (Linear Prediction) method,Min-Norm method, MUSIC method, and ESPRIT method can be exemplified.

Procedures of Location Estimating Processing

Next, the procedures of the location estimating processing in thereader/writer 2 will be described with reference to the flowcharts ofFIGS. 18 and 19.

First, when the location estimating processing is started, in S91, thefrequency controller 7A controls the PLL section 5A to adjust thefrequency of the carrier signal used to transmit the R/W request signalso as to become the first frequency f₁.

Next, the transmission controller 7B controls the modulator 5B tosuperimpose the data representing the R/W request signal to the carriersignal. The transmission signal modulated by the modulator 5B isamplified by the power amplifier 5C and is then outputted from thetransmitter antenna 3 (S92). Following the transmission of the R/Wrequest signal, the CW (continuous carrier wave) is transmitted via thefirst frequency f₁ (S93).

When detecting the R/W request signal, the RFID tag 1 transmits, as areply, the tag response signal via a carrier frequencies correspondingto the first frequency f₁ of the CW (continuous carrier wave) that isdetected subsequent to the R/W request signal. The tag response signalis received by the receiver antenna 4. At this moment, the first antennaelement 4A is selected by the selector 6D. Therefore, on the basis ofthe signal received by the first antenna element 4A, the receptionprocessor 6 performs a reception processing (S94); and the phaseinformation acquirer 8A performs a phase information acquiringprocessing (S95).

That is, in the reception processor 6, on the basis of Formulas 4 to 6,the frequency converter 6B calculates the I signal and the Q signal bymultiplying the reception signals inputted from the amplifier 6A and thecarrier signal outputted from the PLL section 5A. When acquiring the Isignal and the Q signal from the frequency converter 6B, on the basis ofFormulas 7 and 8, the phase information acquirer 8A calculates the phasechange amounts φ₁ and φ₂ attributable to the first frequency f₁ andstores the amounts and the frequency (first frequency f₁) used as thecarrier signal in a table in a correlated manner (S95).

When the reception processor 6 finishes the reception of the tagresponse signal from the RFID tag 1 (S96), the phase informationacquirer 8A finishes the phase information acquiring processing (S97).Thereafter, the transmission processor 5 finishes the transmission ofthe CW (continuous carrier wave), that is, the transmission of theone-frame signal (S98). The reception controller 7C determines whetherthe reception signals has been received for the entire frequencies. Ifit is determined that the reception signals has not been received forthe entire frequencies (NO in S99), the procedure goes back to theprocessing of S91. If it is determined that the reception signals hasbeen received for the entire frequencies (YES in S99), the distancecalculator 8B performs a distance calculation processing (S100) and theprocedure goes to the processing of S101. In this case, the locationcalculator 8 may calculate the distance by using the above-describedMUSIC method as well as on the basis of the phase information. Inaddition, the distance calculation may be performed before and/or afterS111 and S112.

In S101, the selector 6D is switched over to select the second antennaelement 4B. Under the control of the transmission controller 7B, the R/Wrequest signal and the CW (continuous carrier wave) are transmitted viathe first frequency f₁ (S102 to S104). When detecting the R/W requestsignal, the RFID tag 1 transmits, as a reply, the tag response signalvia a carrier frequencies corresponding to the first frequency f₁ of theCW (continuous carrier wave) that is detected subsequent to the R/Wrequest signal. The tag response signal is received by the receiverantenna 4. At this moment, the second antenna element 4B is selected bythe selector 6D. Therefore, on the basis of the signal received by thesecond antenna element 4B, the reception processor 6 performs areception processing (S105).

That is, in the reception processor 6, on the basis of Formulas 4 to 6,the frequency converter 6B calculates the I signal and the Q signal bymultiplying the reception signals inputted from the amplifier 6A and thecarrier signal outputted from the PLL section 5A and outputs the Isignal and the Q signal to the phase information acquirer 8A. When thereception processor 6 finishes the reception of the tag response signalfrom the RFID tag 1 (S106), the transmission processor 5 finishes thetransmission of the CW (continuous carrier wave), that is, thetransmission of the one-frame signal (S107).

Next, processings of S108 and S109 related to the direction calculationprocessing are performed.

In S108, the phase information acquirer 8A detects the phase differencebetween the signal received by the first antenna element 4A and thesignal received by the second antenna element 4B. Then, the directioncalculator 8C calculates the direction of existent location (directionof existence) of the RFID tag 1 on the basis of the phase difference(S109). In the case of estimating the direction on the basis of thephase difference between antenna elements, it is required to compare thephase differences at the same frequency.

Thereafter, the reception controller 7C acquires the distanceinformation calculated by the distance calculator 8B and informationabout the direction of existent location calculated by the directioncalculator 8C and transfers the information to the area determinant 10.The area determinant 10 calculates the location of the RFID tag 1 on thebasis of the distance information and the information about thedirection of existent location (S110). In this manner, the locationmeasurement processing is completed.

Simultaneous Transmission of Multiple Frequencies within One Frame

In the above example, the reader/writer 2 is configured to calculate thedistance such that the carrier frequencies is changed more than onetimes in the course of transmitting the R/W request signal and CW(continuous carrier wave) that is composed of one frame, and the tagresponse signal that is composed of one frame is received with thecarrier frequencies changed more than one times in the midway, therebycalculating the distance on the basis of the tag response signal. To thecontrary, the reader/writer 2 may be configured to calculate thedistance such that the carrier frequencies used in transmitting the R/Wrequest signal and CW (continuous carrier wave) that is composed of oneframe is constituted by a plurality of frequency components, and the tagresponse signal that is composed of one frame is received with thecarrier frequencies having a plurality of frequency components, therebycalculating the distance on the basis of the tag response signal.

The reader/writer 2 is always transmitting a specific signal. Whenrequesting the RFID tag 1 to transmit the tag response signal, as shownin FIG. 20A, the reader/writer 2 transmits the R/W request signal thatrequests a reply of the tag response signal. The frequency controller 7Acontrols the PLL section 5A such that the carrier frequencies of the R/Wrequest signal (more specifically, in the transmission period of the CW(continuous carrier wave) following the R/W request signal) isconstituted by a plurality of frequency components. In the example shownin FIG. 20A, it is controlled such that the carrier frequencies isconstituted by a first frequency f₁, a second frequency f₂, and a thirdfrequency f₃.

The RFID tag 1 is always monitoring the signals sent from thereader/writer 2. When detecting reception of the R/W request signal, theRFID tag 1 transmits the tag response signal in the form of respondingto the R/W request signal. In this case, the tag response signal istransmitted via a carrier frequencies corresponding to the carrierfrequencies having a plurality of frequency components of the R/Wrequest signal (more specifically, the CW (continuous carrier wave)following the R/W request signal). In the example shown in FIG. 20B, thecarrier frequencies of the tag response signal are configured such thatfrequency f₁ is used in the first period, frequency f₂ is used in thesecond period, and frequency f₃ is used in the first period.

By receiving such tag response signal, the location measure 8 becomespossible to detect the states of the reception signals transmitted via aplurality of different frequencies.

Hereinafter, the configuration for enabling such detection will bedescribed with reference to FIG. 21.

The configuration shown in FIG. 21 is different from that of FIG. 4 inthat the transmission processor 5 of FIG. 21 is provided withtransmitters 5D1, 5D2, and 5D3 corresponding to each frequency and acombiner 5E, and the reception processor 6 is provided with band-passfilters 6C1, 6C2, and 6C3 and mixers 6B3, 6B4, and 6B5 as the frequencyconverter 6B. Other configurations are the same as those of FIG. 4, andthus descriptions thereof will be omitted.

In the communication controller 7, the frequency controller 7A controlsthe PLL section 5A such that the carrier frequencies of the R/W requestsignal (more specifically, in the transmission period of the CW(continuous carrier wave) following the R/W request signal) isconstituted by a plurality of frequency components. The carrier signalsgenerated by the PLL section 5A and the transmitters 5D1, 5D2, and 5D3are combined by the combiner 5E and are transmitted through themodulator 5B and the power amplifier 5C.

The reception signals received at the receiver antenna 4 are branchedinto three paths after passing the amplifier 6A and are respectivelyinputted to the band-pass filters 6C1, 6C2, and 6C3. From the receptionsignals inputted to the band-pass filters 6C1, 6C2, and 6C3, a signal ofa specific frequency component is extracted and inputted to the mixers6B3, 6B4, and 6B5.

The I signal and the Q signal of each frequency components obtained bythe mixers 6B3, 6B4, and 6B5 are inputted to the phase informationacquirer 8A, and the phase change amount of each frequency component andthe distance corresponding to the amount are calculated.

Similar to the examples of FIGS. 4 and 13, the reception processor 6calculates the phase change amount of each frequency component when thereception processor 5 is transmitting the CW (continuous carrier wave).

In the above example, the distance r is calculated on the basis ofphase. However, the same can be applied to the distance measurementprocessing employing the MUSIC method. In addition, a configuration ispossible in which two reception signals of which the S/N ratio and thesignal level of the I and Q signals are higher than those of otherreception signals are selected, and the detection of the phase changeamount and the location calculation are performed on the basis of thetwo reception signals selected.

Active RFID Tag

In the above example, the RFID tag 1 has been described with respect tothe configuration of the passive one. As described above, however, theRFID tag 1 may have the configuration of the active one. In this case,the RFID tag 1 may have a configuration in which the RFID tag 1 isprovided with a battery section, a signal generator that generates thetag response signal, and a frequency controller that is configured totransmit the tag response signal generated by the signal generator via aplurality of different carrier frequencies. In this case, it is possibleto obviate the necessity of the reader/writer 2 transmitting the R/Wrequest signal.

Similar to the frequency controller 7A, the frequency controller of theRFID tag 1 may be configured to set a plurality of divided periods inthe transmission period of one tag response signal such that differentcarrier frequencies are used for each of the divided periods.Alternatively, the frequency controller may be configured such thatdifferent carrier frequencies are used for each transmission of the tagresponse signal. Further alternatively, the carrier frequencies may beconstituted by a plurality of frequency components.

Application Examples of Communication System Using RFID Tag

Next, specific application examples of the communication system usingthe RFID tag of the present embodiment will be described. FIG. 22 showsan example of a communication system using an RFID tag of the inventionis applied to a system for inspecting and/or checking articles incirculation. In the example shown in the drawing, articles having theRFID tag 1 attached thereto are transported via a plurality of beltconveyors. The articles having the RFID tag 1 attached thereto are alsostacked on places apart from the belt conveyors. The reader/writer 2communicates with the RFID tags 1 in order to inspect the articles beingtransported via the belt conveyors. In this manner, it becomes possibleto manage the circulation of the articles.

In such an communication system using an RFID tag, a managementapparatus that manages the articles in circulation is provided as theexternal apparatus for communicating with the reader/writer 2.

In such a communication system using RFID tag, if the communication areais not definitely defined, the system may uselessly communicates withthe RFID tags 1 located at a position apart from the belt conveyors,which do not need to be communicated with. To the contrary, according tothe communication system using RFID tag of the present embodiment, it ispossible to detect the distance to (or location of) each RFID tag 1 bythe processing of the reader/writer 2. Accordingly, it becomes possibleto communicate with only the RFID tags 1 that need to be communicatedwith.

According to the communication system using RFID tag of the presentembodiment, since the distance to (or location of) each RFID tag 1 canbe detected with relatively high precision, the reader/writer 2 becomespossible to identify the belt conveyor line where the RFID tag 1 isbeing transported.

Only one installation of the reader/writer 2 is sufficient for theconstruction of the communication system using RFID tag having suchfunctions. In other words, it is possible to obviate the necessity of amember for limiting the reachable range of radio waves or the necessityof installing a plurality of readers and/or writers 2. Therefore,according to the RFID tag communication system of the presentembodiment, it becomes possible to perform setting operation in a simplemanner regardless of installing environment.

FIG. 23A shows an example of a system for use in stores, in which thecommunication system using RFID tag of the invention is applied to asystem for surveillance monitoring of products or stored articles. Inthis example, a communication area is set to a range in which articleshaving the RFID tag 1 attached thereto are naturally and originallyexistable. When the article is moved out of the communication area, themovement of the article is detected, thus determining that there is apossibility that the article has been stolen. FIG. 23B shows an exampleof a system in which the RFID tag communication system of the inventionis applied to an security system in which an RFID tag 1 is attached, forexample, to a window or a door; and the position of the RFID tag 1 ismonitored, thereby detecting opening of the window or the door. Theconstruction of such an security system only requires a simple settingoperation, i.e., an operation of attaching the RFID tag 1 to a targetobject, which is conventionally used. Moreover, it becomes possible toconstruct the RFID tag communication system under various environmentsin a flexible manner.

Even in the case of the above example, in the communication system usingRFID tag, a management apparatus that manages the security operations isprovided as the external apparatus for communicating with thereader/writer 2.

FIG. 24 shows an example of a system in which the communication systemusing RFID tag of the invention is applied to the places such asstations or movie theaters where tickets are examined. Nowadays, aticket examination system based on RFID tags is widely used for ticketexamination in stations, for instance. In such a system, the ticketexamination is performed by reader/writers that are provide to gates. Tothe contrary, according to the communication system using RFID tag ofthe present embodiment, by setting the whole passage where the ticketexamination is made as the communication area, it becomes possible toperform the ticket examination without needing to prepare the gates. Inthis case, the RFID tag 1 may be installed in portable phones held byusers.

In the application example shown in FIG. 24, when the result ofcommunication with the RFID tag 1 shows that there is a user who is notallowed to enter, it is conceivable that it is necessary to specify thedisapproved user. In this case, by turning a monitoring camera towardthe location of the disapproved user, as specified by the reader/writer2 so as to capture images of the user, it becomes possible to specifythe disapproved user and to store the images as an evidence.

Even in the case of the above example, in the communication system usingRFID tag, a management apparatus that manages the entrance approvingoperations is provided as the external apparatus for communicating withthe reader/writer 2. The management apparatus controls the capturingoperation of the monitoring camera.

FIG. 25 shows an example of a system in which the communication systemusing RFID tag of the invention is applied to a keyless entry systemthat is suitable for automobiles. The reader/writer 2 is provided in theinterior of an automobile, and a user holds a key having the RFID tag 1incorporated therein. When detecting that the RFID tag 1 incorporatedinto the key held on the user is disposed within a prescribed range of aspace around the automobile, the reader/writer 2 instructs to unlock thekey. With such a configuration, the user can unlock the key only byapproaching the automobile while holding the key thereon.

Even in the case of the above example, in the communication system usingRFID tag, a management apparatus that controls and manages the lockstates of the key is provided as the external apparatus forcommunicating with the reader/writer 2.

INDUSTRIAL APPLICABILITY

A distance measuring apparatus and a communication system equipped withthe tag communication apparatus according to the invention areapplicable to, for example, the system for inspecting and/or checkingarticles in circulation, the system for use in stores for surveillancemonitoring of products, the ticket examination system which is installedat places such as stations or movie theaters where tickets are examined,and the keyless entry system.

Although the invention has been described in detail with reference tospecific embodiments, it will be apparent to those skilled in the artthat various modifications and variations can be made to theabove-described embodiments of the present invention without departingfrom the spirit or scope of the invention.

This application is based on Japanese Patent application No. 2005-066298filed Mar. 9, 2005, the contents of which is incorporated herein byreference in its entirety.

1. A distance measuring apparatus, comprising: a transmitter thattransmits a request signal from an antenna to the outside via radiowaves having a plurality of different carrier frequencies; a receiverthat receives a reflection signal which is a signal composed of oneframe and is obtained when the request signal transmitted from thetransmitter is reflected by a reflector while being modulated; a phaseinformation acquirer that calculates a phase change amount between thereflection signal received by the receiver and the request signal foreach of the carrier frequencies transmitted from the transmitter; and adistance calculator that calculates a distance between the antenna andthe reflector on the basis of the carrier frequencies and the phasechange amount for each of the carrier frequencies acquired by the phaseinformation acquirer, wherein: the receiver comprises a frequencyconverter that converts the received reflection signal to an I signaland a Q signal; the reflection signal is transmitted from the reflectorvia three or more different carrier frequencies; and the phaseinformation acquirer extracts, as a minimal component, one of the Isignal and the Q signal that has a smaller signal level for each of thecarrier frequencies, selects two carrier frequencies having minimalcomponents greater than that of at least one of the other carrierfrequencies, and acquires the phase change amount for each of theselected two carrier frequencies.
 2. The distance measuring apparatus asset forth in claim 1, wherein: the transmitter sets a plurality ofdivided periods in a period for transmitting one request signal, andcontrols such that the different carrier frequencies are used for therespective divided periods.
 3. The distance measuring apparatus as setforth in claim 1, wherein: the transmitter transmits the request signalvia one of the carrier frequencies composed of different carrierfrequency components.
 4. The distance measuring apparatus as set forthin claim 1, wherein: the request signal is a signal composed of oneframe; and the reflection signal includes a preamble section.
 5. Thedistance measuring apparatus as set forth in claim 1, wherein: thedistance calculator calculates the distance with high-resolutionspectrum analysis.
 6. The distance measuring apparatus as set forth inclaim 5, wherein: the distance calculator utilizes a MUSIC (MUltipleSignal Classification) method as the high-resolution spectrum analysis,in which the reflection signal received via the different carrierfrequencies is used as an input, a MUSIC evaluation function is obtainedusing a mode vector as a function of the distance, and the distance iscalculated by obtaining peak values of the MUSIC evaluation function. 7.The distance measuring apparatus as set forth in claim 1, wherein: thedistance calculator calculates the distance also on the basis of areception intensity of the reflection signal received.
 8. The distancemeasuring apparatus as set forth in claim 1, further comprising: areception controller that acquires information in a data section of thereflection signal, and outputs, to the outside, the distance calculatedby the distance calculator which is associated with the information inthe data section.
 9. The distance measuring apparatus as set forth inclaim 1, wherein: the distance calculator measures a direction that thereflector that has transmitted the reflection signal is located, on thebasis of the reflection signal.
 10. The distance measuring apparatus asset forth in claim 1, wherein: the distance calculator calculates thedistance by analyzing a signal in a preamble section of the reflectionsignal.
 11. A communication system, comprising: the distance measuringapparatus as set forth in claim 1; and at least one reflector thatperforms wireless communication with the distance measuring apparatus.12. A communication system, comprising: the distance measuring apparatusas set forth in claim 1; and a management apparatus, that manages atleast one of articles, people, and living things that are associatedwith the reflectors on the basis of a communication result between thedistance measuring apparatus and the reflectors.
 13. A distancemeasuring method, comprising: a transmitting step of transmitting arequest signal from an antenna to the outside via radio waves having aplurality of different carrier frequencies; a receiving step ofreceiving a reflection signal which is obtained when the request signaltransmitted in the transmitting step is reflected from a reflector whilebeing modulated; a phase information acquisition step of calculating aphase change amount between the reflection signal received in thereceiving step and the request signal for each of the carrierfrequencies transmitted in the transmitting step; and a distancecalculating step of calculating a distance between the antenna and thereflector on the basis of the carrier frequencies and the phase changeamount for each of the carrier frequencies acquired in the phaseinformation acquiring step, wherein: the receiving step comprises afrequency converting step of converting the received reflection signalto an I signal and a Q signal; the reflection signal is transmitted fromthe reflector via three or more different carrier frequencies; and thephase information acquisition step includes: a step of extracting, as aminimal component, one of the I signal and the Q signal that has asmaller signal level for each of the carrier frequencies, a step ofselecting two carrier frequencies having minimal components greater thanthat of at least one of the other carrier frequencies, and a step ofacquiring the phase change amount for each of the selected two carrierfrequencies.
 14. The distance measuring method as set forth in claim 13,wherein: the transmitting step is configured to set a plurality ofdivided periods in a period for transmitting one request signal, and tocontrol such that the different carrier frequencies are used for therespective divided periods.
 15. The distance measuring method as setforth in claim 13, wherein: the transmitting step is configured totransmit the request signal via one of the carrier frequencies composedof different carrier frequency components.
 16. The distance measuringmethod as set forth in claim 13, wherein: the request is a signalcomposed of one frame; and the reflection signal includes a preamblesection.
 17. The distance measuring method as set forth in claim 13,wherein: the distance calculating step calculates the distance withhigh-resolution spectrum analysis.
 18. The distance measuring method asset forth in claim 17, wherein: the distance calculator utilizes a MUSIC(Multiple Signal Classification) method as the high-resolution spectrumanalysis, in which the reflection signal received via the differentcarrier frequencies is used as an input, a MUSIC evaluation function isobtained using a mode vector as a function of the distance, and thedistance is calculated by obtaining peak values of the MUSIC evaluationfunction.
 19. The distance measuring method as set forth in claim 13,wherein: the distance calculating step calculates the distance on thebasis of a reception intensity of the reflection signal received. 20.The distance measuring method as set forth in claim 13, furthercomprising: a reception control step of acquiring information in a datasection of the reflection signal, and outputting, to the outside, thedistance calculated by the distance calculator which is associated withthe information in the data section.
 21. The distance measuring methodas set forth in claim 13, wherein: the distance calculating stepmeasures a direction that the reflector that has transmitted thereflection signal is located, on the basis of the reflection signal. 22.The distance measuring method as set forth in claim 13, wherein: thedistance calculating step calculates the distance by analyzing a signalin a preamble section of the request signal.